dm: gpio: Don't use the driver model uclass for SPL

Driver model does not support SPL yet, so we should not use the GPIO
uclass for SPL.

Signed-off-by: Simon Glass <sjg@chromium.org>

drivers/gpio/Makefile

@@ -5,7 +5,9 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
+ifndef CONFIG_SPL_BUILD
 obj-$(CONFIG_DM_GPIO)		+= gpio-uclass.o
+endif
 
 obj-$(CONFIG_AT91_GPIO)	+= at91_gpio.o
 obj-$(CONFIG_INTEL_ICH6_GPIO)	+= intel_ich6_gpio.o