arm: mvebu: Flush caches and disable MMU only on A38x

arm: mvebu: Flush caches and disable MMU only on A38x

Only with disabled MMU its possible to switch the base register address
on Armada 38x. Without this the SDRAM located at >= 0x4000.0000 is also
not accessible, as its still locked to cache.

So to fully release / unlock this area from cache, we need to first
flush all caches, then disable the MMU and disable the L2 cache.

On Armada XP this does not seem to be needed. Even worse, with this
code added, I sometimes see strange input charactes loss from the
console.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>