ARM: at91: ma5d4: Boot from MMC2 when using SAM-BA

Continue loading U-Boot from MMC2 when the SPL was loaded using SAM-BA
loader. This allows the board to boot system from the removable media
instead of the eMMC, which is useful for commissioning purposes. When
booting from the eMMC, always boot from it as it is not possible to
boot from the SD interface directly.

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