ppc4xx: Add 44x cache locking to better support init-ram in d-cache

This patch adds support for locking the init-ram/stack in d-cache,
so that other regions may use d-cache as well

Note, that this current implementation locks exactly 4k of d-cache,
so please make sure that you don't define a bigger init-ram area. Take
a look at the lwmon5 440EPx implementation as a reference.

Signed-off-by: Stefan Roese <sr@denx.de>