ARM: at91: ma5d4: Switch environment start to eMMC

The redesigned version of the SoM which was released onto the market
does no longer contain SPI flash, but boots from the eMMC. Move the
environment storage to the eMMC.

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