nissa/nereid: implement pd_check_vbus_level
nissa/nereid: implement pd_check_vbus_level

This function is used by USB-C code to poll whether
VBUS is present on the port, and is polled in some states
so it uses the cached vbus present bit provided by the
charger (and actuated via interrupts) rather than doing a
I2C transaction to read the analog voltage on each call.

BUG=b:218747760
TEST=Nereid operates normally
BRANCH=none

Signed-off-by: Peter Marheine <ef0b2c492edde94778d7465bfbaeed52c5be6858@chromium.org>
Change-Id: Iad45c084727bd4b148de423d64c92955290535c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3459584
Reviewed-by: Andrew McRae <44ad1424cfc8411c96d9181798ca41ae55709dfb@google.com>

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