arch: riscv: pmp: simplify thread initialization
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 <1635390bb889c4cdee38f53a57b0915ac07d9bd9@andestech.com>

galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr