riscv: Use generic pgprot_* macros from <linux/pgtable.h>

The <linux/pgtable.h> header now defines generic pgprot_ macros also for
the no-MMU configuration, so let's use them.

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>