soc/apollolake: Add minimal GPIO driver

This adds the minimal functionality needed to configure SoC pads.

Change-Id: I2e2268eee2b8c822b42a48a95604b0fab86c9833
Signed-off-by: Andrey Petrov <andrey.petrov@intel.com>
Reviewed-on: https://review.coreboot.org/13676
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)

src/soc/intel/apollolake/Makefile.inc

@@ -9,11 +9,14 @@ subdirs-y += ../../../cpu/x86/tsc
 
 bootblock-y += bootblock/bootblock.c
 bootblock-y += bootblock/cache_as_ram.S
+bootblock-y += gpio.c
 bootblock-y += placeholders.c
 
 romstage-y += placeholders.c
+romstage-y += gpio.c
 smm-y += placeholders.c
 ramstage-y += placeholders.c
+ramstage-y += gpio.c
 
 CPPFLAGS_common += -I$(src)/soc/intel/apollolake/include