powerpc/85xx: Bump up the CONFIG_SYS_BOOTM_LEN to 64M on FSL 85xx boards

CONFIG_SYS_BOOTMAPSZ has been 64M on these boards for some time so we
should also allow the kernel image to be up to 64M decompressed.  This
also matches what we pass to the OS based on the ePAPR specification.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>