cr50: change uart rx to wake_low
cr50: change uart rx to wake_low

Having cr50 resume on the falling edge of its uart rx signal is causing
some issues, and wake on low is good enough and works fine. This change
switches uart rx from DIO_WAKE_FALLING to DIO_WAKE_LOW

BUG=chrome-os-partner:60449
BRANCH=none
TEST=cr50 can still resume on uart activity and plugging in the charger
has no impact on cr50 remaining in deep sleep.

Change-Id: If77126cb64cf2fa949a75d53bb40098f037a2aa4
Signed-off-by: Mary Ruthven <81380f3403fed74252e92df3ed012a1819ec6296@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/418335
Reviewed-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>

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