dts: arm: xmc4xxx: Add extra pinctrl nodes for i2c

The existing i2c sda/slc pinctrl nodes serve as an input and output
for the USIC. This limits the number of pins that can be used for i2c
since the pin must be internally connected to both DOUT0 and DX0 signals
on the USIC (for the sda signal for example).

It is also possible to use separate pins to DOUT0 and DX0, but connect
the pins externally. Add these extra pinctrl nodes and document their
use in infineon,xmc4xxx-i2c.yaml.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>