This architecture is a bit strange. It has up to four SDRAM banks, but
all have a quite limited size. The SDRAM size for the different boards
currently is unknown as it's configurable with Kconfig. We use a SDRAM
size based on the value of the only board we have in the defconfigs:
edb9301. This likely breaks other ep93xx boards.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>