mm: pgtable: Make generic pgprot_* macros available for no-MMU

The <linux/pgtable.h> header defines some generic pgprot_*
implementations, but they are only available when CONFIG_MMU is enabled.
The RISC-V architecture, for example, therefore defines some of these
pgprot_* macros for !NOMMU.

Let's make the pgprot_* generic available even for !NOMMU so we can
remove the RISC-V specific definitions.

Compile-tested with x86 defconfig, and riscv defconfig and !MMU defconfig.

Suggested-by: Palmer Dabbelt <palmerdabbelt@google.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>