cortex-m: allow to lock/unlock empty MPU region
cortex-m: allow to lock/unlock empty MPU region

When the MPU region to modify is empty (size == 0) returns EC_SUCCESS
rather than EC_ERROR_INVAL, so we are not failing MPU activation on
systems without anything in the .iram.text section.

Signed-off-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

BRANCH=all
BUG=chrome-os-partner:35774
TEST=on Samus, with WP, try "sysjump disable" on the EC command line
and see "RAM locked. Exclusion 20006ee0-20006ee0" message.

Change-Id: I0103e0f7aa81ee64a70dd47ffb1c50067ce5a6ee
Reviewed-on: https://chromium-review.googlesource.com/242690
Trybot-Ready: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Tested-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>
Commit-Queue: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

fourier49/BZ_DEV_EC,coreboot/chrome-ec,coreboot/chrome-ec,md5555/ec,md5555/ec,md5555/ec,eatbyte/chromium-ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,fourier49/BZ_DEV_EC,fourier49/BIZ_EC,eatbyte/chromium-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,fourier49/BIZ_EC,coreboot/chrome-ec,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,md5555/ec,fourier49/BIZ_EC,eatbyte/chromium-ec,fourier49/BIZ_EC,akappy7/ChromeOS_EC_LED_Diagnostics,eatbyte/chromium-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics