RISC-V: mark hibernation as nonportable

Hibernation support depends on firmware marking its reserved/PMP
protected regions as not accessible from Linux.
The latest versions of the de-facto SBI implementation (OpenSBI) do
not do this, having dropped the no-map property to enable 1 GiB huge
page mappings by the kernel.
This was exposed by commit 3335068f ("riscv: Use PUD/P4D/PGD pages
for the linear mapping"), which made the first 2 MiB of DRAM (where SBI
typically resides) accessible by the kernel.
Attempting to hibernate with either OpenSBI, or other implementations
following its lead, will lead to a kernel panic ([1], [2]) as the
hibernation process will attempt to save/restore any mapped regions,
including the PMP protected regions in use by the SBI implementation.

Mark hibernation as depending on "NONPORTABLE", as only a small subset
of systems are capable of supporting it, until such time that an SBI
implementation independent way to communicate what regions are in use
has been agreed on.

As hibernation support landed in v6.4-rc1, disabling it for most
platforms does not constitute a regression. The alternative would have
been reverting commit 3335068f

 ("riscv: Use PUD/P4D/PGD pages for
the linear mapping").
Doing so would permit hibernation on platforms with these SBI
implementations, but would limit the options we have to solve the
protection of the region without causing a regression in hibernation
support.

Reported-by: Song Shuai <suagrfillet@gmail.com>
Link: https://lore.kernel.org/all/CAAYs2=gQvkhTeioMmqRDVGjdtNF_vhB+vm_1dHJxPNi75YDQ_Q@mail.gmail.com/ [1]
Reported-by: JeeHeng Sia <jeeheng.sia@starfivetech.com>
Link: https://groups.google.com/a/groups.riscv.org/g/sw-dev/c/ITXwaKfA6z8 [2]
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230526-astride-detonator-9ae120051159@wendy
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>