riscv: mm: mark noncoherent_supported as __ro_after_init

The noncoherent_supported indicates whether the HW is coherent or not,
it won't change after booting, mark it as __ro_after_init.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230614165504.532-4-jszhang@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>