RISC-V: Add EFI stub support.

RISC-V: Add EFI stub support.

Add a RISC-V architecture specific stub code that actually copies the
actual kernel image to a valid address and jump to it after boot services
are terminated. Enable UEFI related kernel configs as well for RISC-V.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Link: https://lore.kernel.org/r/20200421033336.9663-4-atish.patra@wdc.com
[ardb: - move hartid fetch into check_platform_features()
       - use image_size not reserve_size
       - select ISA_C
       - do not use dram_base]
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>