mips: mscc_sgpio: Add the MSCC serial GPIO device (SIO)

mips: mscc_sgpio: Add the MSCC serial GPIO device (SIO)

This add support for the the MSCC serial GPIO driver in MSCC
VCoreIII-based SOCs.

By using a serial interface, the SIO controller significantly extends
the number of available GPIOs with a minimum number of additional pins
on the device. The primary purpose of the SIO controller is to connect
control signals from SFP modules and to act as an LED controller.

This adds the base driver.

Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>