riscv: efi: enable generic EFI compressed boot

Wire up the generic EFI zboot support for RISC-V.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Tested-by: Conor Dooley <conor.dooley@microchip.com>