initramfs: Provide a common initrd reserve function

mainline inclusion
from mainline-v5.12-rc1-dontuse
commit c72160fe
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5OHOB


CVE: NA

--------------------------------

Some architectures(eg, ARM and riscv) have similar logic to
check and reserve the memory of initrd, let's provide a common
function reserve_initrd_mem() to reduce duplicated code.

Change-Id: Icb41095331f45bb820b6a33b0d143745eaea8c05
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>