chip/npcx: Make check_reset_cause() static

This function is not called from outside this file so make it static. Also
drop a pointless comment, and rename the function since the 'system' prefix
suggests it is defined in system.h (which it is not).

BUG=chromium:876737
BRANCH=none
TEST=make -j50 BOARD=grunt
Change-Id: Ic429fb1da2e56e1888e008f4739c90e8ed2c1947
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1184975
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Reviewed-by: Daisuke Nojiri <dnojiri@chromium.org>