riscv: Add pgprot_writecombine/device and PAGE_SHARED defination if NOMMU

Some drivers use PAGE_SHARED, pgprot_writecombine()/pgprot_device(),
add the defination to fix build error if NOMMU.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/mmio.h

@@ -16,6 +16,8 @@
 
 #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. */

arch/riscv/include/asm/pgtable.h

@@ -470,6 +470,7 @@ static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
 
 #else /* CONFIG_MMU */
 
+#define PAGE_SHARED		__pgprot(0)
 #define PAGE_KERNEL		__pgprot(0)
 #define swapper_pg_dir		NULL
 #define VMALLOC_START		0