xtensa: mmu: mpu: add xtensa_mem_kernel_has_access()

This adds a new function xtensa_mem_kernel_has_access() to
determine if a memory region can be accessed by kernel threads.
This allows checking for valid mapped memory before accessing
them to avoid relying on page faults to detect invalid access.

Also fixed an issue with arch_buffer_validate() on MPU where
it may return okay even if the incoming memory region has no
corresponding entry in the MPU table.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>