nissa/nereid: implement VBUS sourcing
nissa/nereid: implement VBUS sourcing

pd_set_power_supply_ready is the board function that turns on VBUS
when requested by the PD state machine; implement it for Nereid.

BUG=b:201000844
TEST=builds
BRANCH=none

Signed-off-by: Peter Marheine <ef0b2c492edde94778d7465bfbaeed52c5be6858@chromium.org>
Change-Id: I6b092080ba383b6d48767ed6c4e8341a1b13e35a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3478974
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