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>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index c8c6c60..aa55ff4 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -106,6 +106,17 @@
 	  Support controlling the GPIO used for SPI bitbang by software. Can
 	  be used by the VCoreIII SoCs, but it was mainly useful for Luton.
 
+config MSCC_SGPIO
+	bool "Microsemi Serial GPIO driver"
+	depends on DM_GPIO && SOC_VCOREIII
+	help
+	  Support for the VCoreIII SoC serial GPIO device. 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.
+
 config MSM_GPIO
 	bool "Qualcomm GPIO driver"
 	depends on DM_GPIO