ARM: sama5d2: Implement boot device autodetection

Implement support for saving ARM register R4 early during boot using
save_boot_params . Implement support for decoding the stored register
R4 value in spl_boot_device() to obtain boot device from which the
SoC booted. This way, the SPL will always load U-Boot from the same
device from which the SPL itself booted instead of using hard-coded
boot device.

This functionality is useful for example when booting sama5d2-xplained
from SD card, where by default the SPL would try loading the U-Boot
from eMMC and fail. This is because eMMC is on SDHCI0 (BOOT_DEVICE_MMC1),
while SD slot is on SDHCI1 (BOOT_DEVICE_MMC2) and the SPL was hard-wired
to always boot from BOOT_DEVICE_MMC1.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Andreas Bießmann <andreas@biessmann.org>
diff --git a/arch/arm/mach-at91/Makefile b/arch/arm/mach-at91/Makefile
index 4424523..d2abf31 100644
--- a/arch/arm/mach-at91/Makefile
+++ b/arch/arm/mach-at91/Makefile
@@ -9,7 +9,7 @@
 obj-$(CONFIG_AT91SAM9M10G45) += mpddrc.o spl_at91.o
 obj-$(CONFIG_AT91SAM9N12) += mpddrc.o spl_at91.o
 obj-$(CONFIG_AT91SAM9X5) += mpddrc.o spl_at91.o
-obj-$(CONFIG_SAMA5D2) += mpddrc.o spl_atmel.o matrix.o atmel_sfr.o
+obj-$(CONFIG_SAMA5D2) += bootparams_atmel.o mpddrc.o spl_atmel.o matrix.o atmel_sfr.o
 obj-$(CONFIG_SAMA5D3) += mpddrc.o spl_atmel.o
 obj-$(CONFIG_SAMA5D4) += mpddrc.o spl_atmel.o matrix.o atmel_sfr.o
 obj-y += spl.o