arch: riscv: pmp: simplify thread initialization

Thread init related to PMP & userspace contains 5 parts:

1. User/supervisor thread clear PMP context
2. User thread clear it's context
3. User/supervisor thread assign to different entry
4. Supervisor thread assign mstatus.MPRV for M-mode PMP protection
5. User/supervisor thread setup PMP regions of stack guard if enabled

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