pd: Changes to USB PD state machine for SRC DTS mode
pd: Changes to USB PD state machine for SRC DTS mode

In order to make pass through charging safe, there must be a mechanism
to signal the CHG port manager that Vbus is available and in a steady
state. When a typec only charger is used for the CHG port, then it's
steady state is SRC_DISCOVERY. Need a way to differentiate the TypeC
only charger from a USB PD charger that's also going through
SNK_DISCOVERY.

In addition, to allow a DTS device to support PD messaging, can't
stay in SRC_ACCESSORY state. This CL makes a modification to the
USB PD state machine so that if a debug accessory is detected and
CONFIG_USB_PD_DTS is defined, then the state machine will advance to
SRC_STARTUP and prepare for messages similar to when a UFP attach
is detected.

BUG=b:35586526
BRANCH=servo
TEST=Manual Built for both Plankton and Reef, verified that Reef
connects as both SNK and SRC.

Change-Id: I9aea0341cd9b6e6773023f0923a9a18a86fc4627
Signed-off-by: Scott <7d40b3eb9e17bf55d972074a5e77a7ea4159f9dd@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/448987
Commit-Ready: Scott Collyer <7d40b3eb9e17bf55d972074a5e77a7ea4159f9dd@chromium.org>
Tested-by: Scott Collyer <7d40b3eb9e17bf55d972074a5e77a7ea4159f9dd@chromium.org>
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

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