ARM: dts: alt: use demuxer for I2C4

Make it possible to fallback to GPIO for I2C4 on the EXIO-B 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 and fixed aliases]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>