src/mb/asus/p5qc/gpio.c: Allow specifying a gpio.c in Kconfig

Even though these two mainboards use the same gpio.c file, other boards
such as the p5ql_pro do not.

Change-Id: I2f7c8c12cb1bdcf47f3b4d4cef0b11e44a5b8863
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/29447
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/mainboard/asus/p5qc/Kconfig

@@ -48,6 +48,10 @@ config DEVICETREE
 	string
 	default "variants/$(CONFIG_VARIANT_DIR)/devicetree.cb"
 
+config GPIO_C
+	string
+	default "gpio.c"
+
 config MAX_CPUS
 	int
 	default 4

src/mainboard/asus/p5qc/Makefile.inc

@@ -11,5 +11,7 @@
 # GNU General Public License for more details.
 #
 
+CONFIG_GPIO_C:=$(call strip_quotes, $(CONFIG_GPIO_C))
+
 ramstage-y += cstates.c
-romstage-y += gpio.c
+romstage-y += $(CONFIG_GPIO_C)