ppc4xx: Fix crash on sequoia with cache enabled

Currently U-Boot crashes on sequoia board in CPU POST if
cache is enabled (CONFIG_4xx_DCACHE defined). The cache
won't be disabled by change_tlb before CPU POST because
there is an insufficient adress range check since
CFG_MEM_TOP_HIDE was introduced. This patch tries to fix
this problem.

Signed-off-by: Anatolij Gustschin <agust@denx.de>