riscv: pmp: don't reconfigure modes

Previously PMP was avaible only with Multithreading, since it's now
available without MT - add extra checks to prevent user/kernel mode
reconfiguration.

Signed-off-by: Volodymyr Fialko <vfialko@marvell.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>