mvebu: axp: Rename MV_DDR_32BIT to CONFIG_DDR_32BIT

This should make it clear that this symbol is meant to be defined by
board headers.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>