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>