Create a separate bus for HDMI related I2C2 and provide fallback to GPIO.

Based on work for the r8a7790/lager by Wolfram Sang.
Signed-off-by: NSimon Horman <horms+renesas@verge.net.au>
[wsa: rebased, corrected board name in subject, fixed aliases, switched
to named GPIOs]
Signed-off-by: NWolfram Sang <wsa+renesas@sang-engineering.com>