riscv: pgtable: Fix __kernel_map_pages build error if NOMMU

riscv64-none-linux-gnu-ld: mm/page_alloc.o: in function `.L0 ':
page_alloc.c:(.text+0xd34): undefined reference to `__kernel_map_pages'
riscv64-none-linux-gnu-ld: page_alloc.c:(.text+0x104a): undefined reference to `__kernel_map_pages'
riscv64-none-linux-gnu-ld: mm/page_alloc.o: in function `__pageblock_pfn_to_page':
page_alloc.c:(.text+0x145e): undefined reference to `__kernel_map_pages'

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>