arch: arm: mpu: enable WBWA caching on per thread user RAM

Update _get_region_attr_by_type used to set up the user regions
on a per thread basis to match the REGION_USER_RAM_ATTR macro. Caching
attributes were set "Outer and inner write-through. No write allocate.",
this patch changes it to "Outer and inner write-back. Write and read
allocate.".

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>