southbridge/intel/common: Add common gpio.c

Add a common southbridge gpio code to reduce existing
duplicated code.
By adding it to ram-stage, GPIOs can be changed any time,
without the need of direct register access.

The files are based on bd82x6x and lynxpoint gpio.c.

Change-Id: Iaf0c2f941f2625a5547f9cba79da1b173da6f295
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/12893
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/southbridge/intel/common/Kconfig

@@ -1,2 +1,4 @@
 config SOUTHBRIDGE_INTEL_COMMON
 	def_bool n
+config SOUTHBRIDGE_INTEL_COMMON_GPIO
+	def_bool n