sh-pfc: r8a7790: add pin definitions for the I2C3 interface

There are four I2C interfaces on r8a7790, each of them can be connected to
one of the two respective I2C controllers, e.g. interface #0 can be
configured to work with I2C0 or with IIC0. Additionally some of those
interfaces can also use one of several pin sets. Interface #3 is special,
because it can be used in automatic mode for DVFS. It only has one set
of pins available and those pins cannot be used for anything else, they
also lack the GPIO function.

This patch uses the sh-pfc ability to configure pins, not associated with
GPIOs and adds support for I2C3 to the r8a7790 PFC set up.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>