arm: mvebu: Add CRS305-1G-4S board

CRS305-1G-4S has a switch chip with an integrated CPU (98DX3236) and
like some of the other similar boards requires bin_hdr.
bin_hdr (DDR3 init stage) is currently retrieved from the stock
bootloader and compiled into the kwb image.

Adds support for U-Boot, enable UART, SPI, Winbond SPI flash chip
support and writing env to SPI flash.

Signed-off-by: Luka Kovacic <me@lukakovacic.xyz>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/board/mikrotik/crs305-1g-4s/binary.0 b/board/mikrotik/crs305-1g-4s/binary.0
new file mode 100644
index 0000000..8dd6872
--- /dev/null
+++ b/board/mikrotik/crs305-1g-4s/binary.0
@@ -0,0 +1,11 @@
+--------
+WARNING:
+--------
+This file should contain the bin_hdr generated by the original Marvell
+U-Boot implementation. As this is currently not included in this
+U-Boot version, we have added this placeholder, so that the U-Boot
+image can be generated without errors.
+
+If you have a known to be working bin_hdr for your board, then you
+just need to replace this text file here with the binary header
+and recompile U-Boot.