mvebu: Migrate CONFIG_DDR_32BIT/64BIT to Kconfig

Move CONFIG_DDR_32BIT/64BIT to Kconfig as a choice for Armada XP
platforms.  Make 64bit the default as this mirrors the current code.

Cc: Marek Behún <marek.behun@nic.cz>
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>