zephyr: Add locking to shimmed flash.c
zephyr: Add locking to shimmed flash.c

Add a mutex to the shimmed version of flash.c (see
chip/npcx/flash.c for reference). While I haven't run into this as
a direct issue yet, this could very possibly be where we were
running into the boot hang prior.

BRANCH=none
BUG=b:180112248, b:179900857, b:167392037
TEST=zmake testall
TEST=build volteer, flash, see that it boots.

Signed-off-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>
Change-Id: I8013eba02545f96406399b8f4966e92bed1e9e9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2686983
Reviewed-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

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