ARM: at91: ma5d4: Support both SF and eMMC SoMs

Discern the SoMs based on the presence of SPI flash to support both
variants of the SoM, one booting from SPI NOR and one booting from
eMMC.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andreas Bießmann <andreas.devel@googlemail.com>