mdss: HDMI: Add support for populating SPD infoframe fields

Expose sysfs nodes that can be used to populate the Vendor
Name and Product Description fields in the Source Product
Description (SPD) infoframe for transmission to an HDMI sink

Change-Id: I7eeeaae5fca0c657eef62273cb17da22146ca516
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>