dm: gpio: Allow control of GPIO uclass in SPL

At present if CONFIG_SPL_GPIO_SUPPORT is enabled then the GPIO uclass
is included in SPL/TPL without any control for boards. Some boards may
want to disable this to reduce code size where GPIOs are not needed in
SPL or TPL.

Add a new Kconfig option to permit this. Default it to 'y' so that
existing boards work correctly.

Change existing uses of CONFIG_DM_GPIO to CONFIG_IS_ENABLED(DM_GPIO) to
preserve the current behaviour. Also update the 74x164 GPIO driver since
it cannot build with SPL.

This allows us to remove the hacks in config_uncmd_spl.h and
Makefile.uncmd_spl (eventually those files should be removed).

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index c1ad5d6..447cf04 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -14,6 +14,28 @@
 	  particular GPIOs that they provide. The uclass interface
 	  is defined in include/asm-generic/gpio.h.
 
+config SPL_DM_GPIO
+	bool "Enable Driver Model for GPIO drivers in SPL"
+	depends on DM_GPIO && SPL_DM && SPL_GPIO_SUPPORT
+	default y
+	help
+	  Enable driver model for GPIO access in SPL. The standard GPIO
+	  interface (gpio_get_value(), etc.) is then implemented by
+	  the GPIO uclass. Drivers provide methods to query the
+	  particular GPIOs that they provide. The uclass interface
+	  is defined in include/asm-generic/gpio.h.
+
+config TPL_DM_GPIO
+	bool "Enable Driver Model for GPIO drivers in TPL"
+	depends on DM_GPIO && TPL_DM && TPL_GPIO_SUPPORT
+	default y
+	help
+	  Enable driver model for GPIO access in TPL. The standard GPIO
+	  interface (gpio_get_value(), etc.) is then implemented by
+	  the GPIO uclass. Drivers provide methods to query the
+	  particular GPIOs that they provide. The uclass interface
+	  is defined in include/asm-generic/gpio.h.
+
 config GPIO_HOG
 	bool "Enable GPIO hog support"
 	depends on DM_GPIO