riscv: introduce ioremap_wc()

Compared with IO attributes, NC attributes can improve performance,
specifically in these aspects: Relaxed Order, Gathering, Supports Read
Speculation, Supports Unaligned Access.

Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
Signed-off-by: Qingfang Deng <qingfang.deng@siflower.com.cn>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250722091504.45974-2-cuiyunhui@bytedance.com


Signed-off-by: Paul Walmsley <pjw@kernel.org>