arm64: mm: add __init section marker to free_initrd_mem

It is not needed after booting, this patch moves the
free_initrd_mem() function to the __init section.

This patch also make keep_initrd __initdata, to reduce kernel
size.

Signed-off-by: Wang Long <long.wanglong@huawei.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>