ec: use symbolic names instead of magic numbers
ec: use symbolic names instead of magic numbers

PD_DP_PIN_CAPS used a lot of magic numbers, which made it difficult
to work out what it's doing. Added a comment about using the
"receptacle type" field to deterimine whether the UFP_D or DFP_D pin
assignments should be used, and replaced magic numbers with #define'd
constants.

BUG=None
BRANCH=None
TEST=`make -j buildall && ./util/flash_ec --board=kohaku` (or
whatever board you're testing with), then verify that a USB-C dock
with HDMI or DisplayPort still works.

Change-Id: I1b5cf6d6cf7d0e1698bd7c727226f10f804ed5e9
Signed-off-by: Paul Fagerburg <d8c44b60afb21c60f56d931aa690ca1cf34910c8@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1935088
Reviewed-by: Jett Rink <9d4aef0593e6843318d99df31597fb6153b89787@chromium.org>

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