gpio: Add Aspeed SGPIO driver

The Aspeed SGPIO driver supports the SGPIO controllers found in the
AST2400, AST2500 and AST2600 BMC SoCs. The implementation is a cut-down
copy of the upstream Linux kernel driver, adapted for u-boot.

Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index 1e57116..92a8597 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -157,6 +157,13 @@
 	  is found in the AST2400, AST2500 and AST2600 BMC SoCs and
 	  provides access to over 200 GPIOs on each chip.
 
+config ASPEED_SGPIO
+	bool "Aspeed SGPIO Driver"
+	help
+	  Say yes here to support the Aspeed serial GPIO driver. The controller
+	  is found in the AST2400, AST2500 and AST2600 BMC SoCs and
+	  provides access to generate serial GPIO signal.
+
 config ASPEED_G7_GPIO
 	bool "Aspeed G7 GPIO Driver"
 	help