ARM: k2g-ice: Add pinmux support for rgmii interface

ARM: k2g-ice: Add pinmux support for rgmii interface

This add pinmux configuration for rgmii interface so that network
driver can be supported on K2G ICE boards. The pinmux configurations
for this are generated using the pinmux tool at
https://dev.ti.com/pinmux/app.html#/default

As this required some BUFFER_CLASS definitions, same is re-used
from the linux defnitions in include/dt-bindings/pinctrl/keystone.h

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>