TCPCI: add ALERT_NONE
TCPCI: add ALERT_NONE

This adds the TCPC_REG_ALERT_NONE constant to explicitly checking if the
alert register has no alerts pending.

BRANCH=none
BUG=b:191531291
TEST=buildall passes

Change-Id: If2d2166fbc422f071a394125fbdbe14cf20f54bc
Signed-off-by: Caveh Jalali <08e24a66aefda7a55ca7d73f8903e0fb16eec763@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2986599
Reviewed-by: Denis Brockus <6b21e8190fb5e373363fb95271367073a16474ef@chromium.org>
Commit-Queue: Denis Brockus <6b21e8190fb5e373363fb95271367073a16474ef@chromium.org>

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