cortex-m: Use MPU REGION_CODE_RAM to lock rollback if needed
cortex-m: Use MPU REGION_CODE_RAM to lock rollback if needed

On chips with only 8 MPUs, we need to lock the rollback with unused
MPU regions. Since REGION_STORAGE2 may be used to lock RW, use
REGION_CODE_RAM instead.

BRANCH=none
BUG=b:155410753
TEST=make -j buildall

Change-Id: Iec0f33b668474ed539809a319bf94d11cb52f64a
Signed-off-by: Yicheng Li <77a205905d8955597c069555e9569c30512f6bd8@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2219578
Reviewed-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec