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>

arch/riscv/include/asm/mmio.h

@@ -14,12 +14,6 @@
 #include <linux/types.h>
 #include <asm/mmiowb.h>
 
-#ifndef CONFIG_MMU
-#define pgprot_noncached(x)	(x)
-#define pgprot_writecombine(x)	(x)
-#define pgprot_device(x)	(x)
-#endif /* CONFIG_MMU */
-
 /* Generic IO read/write.  These perform native-endian accesses. */
 #define __raw_writeb __raw_writeb
 static inline void __raw_writeb(u8 val, volatile void __iomem *addr)