arm: nxp: mpu: Consolidate k64 mpu regions

Reduces the number of mpu regions statically reserved at boot time by
one, giving a total of five. We originally sought to reduce the total to
three: 1 background region with lowest precendence for supervisor r/w, 1
flash region, and 1 sram region. However, the nxp mpu hardware does not
give precedence to any region over another, and thus we cannot revoke
access from the background region with a higher priority region. This
means we cannot support hardware stack protection with a single
background region.

Instead, create two background regions that cover the entire address
space, except for sram.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>