ARM: atmel: add sama5d3 Xplained board support

Add sama5d3 Xplained board support which use Atmel SAMA5D36 SoC.
Now it supports boot from NAND flash and SD/MMC card.
Features support:
  - NAND flash
  - SD/MMC card
  - Two USB hosts
  - Ethernet (one GMAC, one EMAC)

Signed-off-by: Bo Shen <voice.shen@atmel.com>
[reorder boards.cfg]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
diff --git a/boards.cfg b/boards.cfg
index d28e09b..e8d7e0f 100644
--- a/boards.cfg
+++ b/boards.cfg
@@ -278,6 +278,8 @@
 Active  arm         armv7          am33xx      ti              am43xx              am43xx_evm                           am43xx_evm:SERIAL1,CONS_INDEX=1                                                                                                   Lokesh Vutla <lokeshvutla@ti.com>
 Active  arm         armv7          am33xx      ti              ti814x              ti814x_evm                           -                                                                                                                                 Matt Porter <matt.porter@linaro.org>
 Active  arm         armv7          am33xx      ti              ti816x              ti816x_evm                           -                                                                                                                                 -
+Active  arm         armv7          at91        atmel           sama5d3_xplained    sama5d3_xplained_mmc                 sama5d3_xplained:SAMA5D3,SYS_USE_MMC                                                                                              Bo Shen <voice.shen@atmel.com>
+Active  arm         armv7          at91        atmel           sama5d3_xplained    sama5d3_xplained_nandflash           sama5d3_xplained:SAMA5D3,SYS_USE_NANDFLASH                                                                                        Bo Shen <voice.shen@atmel.com>
 Active  arm         armv7          at91        atmel           sama5d3xek          sama5d3xek_mmc                       sama5d3xek:SAMA5D3,SYS_USE_MMC                                                                                                    Bo Shen <voice.shen@atmel.com>
 Active  arm         armv7          at91        atmel           sama5d3xek          sama5d3xek_nandflash                 sama5d3xek:SAMA5D3,SYS_USE_NANDFLASH                                                                                              Bo Shen <voice.shen@atmel.com>
 Active  arm         armv7          at91        atmel           sama5d3xek          sama5d3xek_spiflash                  sama5d3xek:SAMA5D3,SYS_USE_SERIALFLASH                                                                                            Bo Shen <voice.shen@atmel.com>