Revert "RISC-V: mark hibernation as nonportable"

Revert commit ed309ce5 ("RISC-V: mark hibernation as nonportable")
as it appears the broken versions of OpenSBI have not made it to
production on any systems that support hibernation.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230802-chef-throng-d9de8b672a49@wendy


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>