ARM: dts: lager: use demuxer for IIC1/I2C1

Make it possible to select which I2C1 IP core you want to run on the
EXIO-A connector.

This is based on reference work for the I2C0 core of the lager board
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>