__flush_dcache_icache_phys() requires the ability to access the
memory with the MMU disabled, which means that on a 32-bit system
any memory above 4 GiB is inaccessible.  In particular, mpc86xx is
32-bit and can have more than 4 GiB of RAM.
Signed-off-by: NScott Wood <scottwood@freescale.com>