pd: support for dual-role port

Add support for toggling between source and sink as dual-role
port. When transitioning to S0, we turn toggling on, when transitioning
to S3, we turn toggling off but remain in the same PD state, and when
transitioning to S5, we turn toggling off and force the PD role to a
sink.

Note, when toggling is off, the source disconnected state is
allowed to transition to sink disconnected, but not vice versa. This
means that if you go into S3 as a source, it will remain a source
until the device is unplugged, at which point it will transition to
a sink until the next transition to S0.

The spec specifies:
tDRP: 50ms - 100ms, Period a DRP shall complete a DFP to UFP and back
dcDRP: 30% - 70%, Percent of time that a DRP shall advertise DFP
tDRPHold: 100ms - 150ms, time to hold VBUS on after a DRP detects a UFP
tDRPLock: 100ms - 150ms, time to stay in DFP after detecting loss of UFP

This CL uses 40ms for time as a UFP (sink), 30ms for time as a DFP
(source), and 120ms for hold and lock times.

Also, if advertising as a DFP (source) and VBUS is detected, this
automatically switches to a UFP (sink).

BUG=chrome-os-partner:28782
BRANCH=none
TEST=test on samus, make sure we are toggling between source and sink
when disconnected. make sure plugging in zinger switches state machine
through to sink_ready and make sure plugging in a USB switches to
source_discovery. tested on a fruitpie by scoping the CC line and verifying
the timing (except the hold time which I can't easily test).

tested that dual role toggling is off in s3 and s5. also verified that
going into s3 as a source keeps the port as a source and going into s5
switches it to a sink.

Change-Id: I478634861f694164301d71359da35142ee7ebf75
Signed-off-by: Alec Berg <alecaberg@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/207154
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>

include/usb_pd.h

@@ -132,6 +132,23 @@ enum pd_errors {
 /* USB Vendor ID assigned to Google Inc. */
 #define USB_VID_GOOGLE 0x18d1
 
+/* --- Protocol layer functions --- */
+
+#ifdef CONFIG_USB_PD_DUAL_ROLE
+enum pd_dual_role_states {
+	PD_DRP_TOGGLE_ON,
+	PD_DRP_TOGGLE_OFF,
+	PD_DRP_FORCE_SINK
+};
+/**
+ * Set dual role state, from among enum pd_dual_role_states
+ *
+ * @param dr_state New state of dual-role port, selected from
+ *                 enum pd_dual_role_states
+ */
+void pd_set_dual_role(enum pd_dual_role_states dr_state);
+#endif
+
 /* --- Policy layer functions --- */
 
 /**