arch: riscv: pmp: introduce riscv_pmp_region structure

Using struct riscv_pmp_region to modulize PMP CSR handling, including
PMP NAPOT/TOR mode handling. This patch can make us more easily to
add/remove RISC-V PMP regions without considering register handling.

Signed-off-by: Jim Shu <cwshu@andestech.com>