arm: mvebu: Add v7_outer_cache_disable function for AXP & A38x

Add functionality to correctly disable the L2 cache on the Armada XP
and 38x platforms.

Without this, booting into Linux on ClearFog (A38x) results in a hangup
without any output on the serial console at all. Even with earlyprintk
enabled.

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