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: Simon Horman <horms+renesas@verge.net.au>
[wsa: rebased, fixed aliases, switched to named GPIOs, sorted by SCL
pins]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>