cortex-m0: add more constraints on atomic implementation
cortex-m0: add more constraints on atomic implementation

In ARMv6-m instruction set, the load/store address register can only be
a "low" register : r0..r7.
Update the inline assembly constraints to match the hardware.

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

BRANCH=none
BUG=none
TEST=make buildall

Change-Id: I9872aeb437b2bb6401bed8076348e26d434320dd
Reviewed-on: https://chromium-review.googlesource.com/224582
Reviewed-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Reviewed-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Commit-Queue: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Tested-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

fourier49/BIZ_EC,akappy7/ChromeOS_EC_LED_Diagnostics,longsleep/ec,md5555/ec,md5555/ec,eatbyte/chromium-ec,longsleep/ec,coreboot/chrome-ec,eatbyte/chromium-ec,coreboot/chrome-ec,fourier49/BIZ_EC,fourier49/BZ_DEV_EC,eatbyte/chromium-ec,longsleep/ec,coreboot/chrome-ec,mtk09422/chromiumos-platform-ec,mtk09422/chromiumos-platform-ec,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,md5555/ec,fourier49/BIZ_EC,eatbyte/chromium-ec,fourier49/BZ_DEV_EC,longsleep/ec,coreboot/chrome-ec,mtk09422/chromiumos-platform-ec,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,md5555/ec,fourier49/BIZ_EC,fourier49/BZ_DEV_EC,mtk09422/chromiumos-platform-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec