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>

cpu/ppc4xx/cache.S

@@ -166,9 +166,11 @@ _GLOBAL(invalidate_dcache)
 #ifdef CONFIG_440
 
        .globl  dcache_disable
+       .globl  dcache_enable
        .globl  icache_disable
        .globl  icache_enable
 dcache_disable:
+dcache_enable:
 icache_disable:
 icache_enable:
 	blr