Make it possible to fallback to GPIO for I2C4 on the EXIO-E connector.

This is based on reference work for the I2C0 core of the lager/r8a7790
by Wolfram Sang.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
[wsa: rebased, corrected board name in subject, fixed aliases, switched
to named GPIOs, fixed pinmux for I2C4]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>