efi: Rename arm-init to efi-init common for all arch

arm-init is responsible for setting up efi runtime and doesn't actually
do any ARM specific stuff. RISC-V can use the same source code as it is.

Rename it to efi-init so that RISC-V can use it.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Link: https://lore.kernel.org/r/20200819222425.30721-8-atish.patra@wdc.com
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>