UPSTREAM: usb: typec: altmodes/displayport: Add hpd sysfs attribute

Exporsing HotPlugDetect(HPD) helps userspace to infer HPD
state as defined by VESA DisplayPort Alt Mode on USB Type-C Standard.
This allows userspace to notify users for self help, for instance,
to hint user that the display port cable is probably detached (or)
the display port sink (viz., monitors ect.,) is un-powered.
Also helps to debug issues reported from field.

This change adds an additional attribute "hpd" to the existing
"displayport" attributes.

VESA DisplayPort Alt Mode on USB Type-C Standard defines how
HotPlugDetect(HPD) shall be supported on the USB-C connector
when operating in DisplayPort Alt Mode. This is a read only
node which reflects the current state of HPD.

Valid values:
 - 1 when HPD’s logical state is high (HPD_High)
 - 0 when HPD’s logical state is low (HPD_Low)

Bug: 253534975
Bug: 260915739
Change-Id: Id72e8ef6ede84038479649c2b753acdac547dea1
Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/lkml/20221211193755.1392128-1-badhri@google.com/T/
(cherry picked from commit 001b0c78)