arch: arm: mpu: fix outer inner WBWA non-shareable macro

Fix NORMAL_OUTER_INNER_WRITE_BACK_WRITE_READ_ALLOCATE_NONSHAREABLE, the
bufferable bit should be set, while the shareable one should not be set.

At the same time, rename it from ..._NONSHAREABLE to _NON_SHAREABLE to
keep the same naming convention for all macro. Given it's not (yet)
used, it should not be an issue.

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