ARM: Use mmap for zImage initrd

We use a large initrd that maxes out our available RAM when loading
kexec. The problem can be mitigated by using slurp_file_mmap(), which
avoids creating a copy of the initrd. The initrd does not use free,
realloc, etc, so it should be safe to use.

Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>
Signed-off-by: Simon Horman <horms@verge.net.au>