ppc4xx: Add dcache_enable() for 440

dcache_enable() was missing for 440 and the patch
017e9b79 ["allow ports to override bootelf
"] behavior uses this function.

Note: Currently the cache handling functions like
d/icache_disable/enable() are NOP's on 440. This may be changed in the
future.

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