qemu-arm: set CONFIG_SYS_BOOTM_LEN to SZ_64M

FIT image contents can be larger than default bootm limit 8M
with initrd. Raise limit to 64MB which is commonly used
elsewhere.

Signed-off-by: Dhananjay Phadke <dphadke@linux.microsoft.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>