power/rk3399: Support USB wake in host-requested wakeable S3
power/rk3399: Support USB wake in host-requested wakeable S3

For boards with POWER_SEQUENCING_VERSION = 2 (and likely future
versions), allow the host to request "wakeable suspend", which will
leave rails enabled to allow wake-on-usb.

BUG=b:63037490
BRANCH=kevin
TEST=With subsequent commit, compile on scarlet w/ power sequencing
version = 2.

Signed-off-by: Shawn Nematbakhsh <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>

Change-Id: Iaadd371b1d1509d185c8c8306b72760dcfe9989f
Reviewed-on: https://chromium-review.googlesource.com/572211
Commit-Ready: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Tested-by: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Reviewed-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>
Reviewed-by: Philip Chen <3ee404f9ef892bc3fda6e6a76e65843b5fa44db2@chromium.org>

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