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/i2c/muxes/pca954x.c b/drivers/i2c/muxes/pca954x.c
index a630ce9..bb2935f 100644
--- a/drivers/i2c/muxes/pca954x.c
+++ b/drivers/i2c/muxes/pca954x.c
@@ -125,7 +125,7 @@
 
 static int pca954x_probe(struct udevice *dev)
 {
-	if (IS_ENABLED(CONFIG_DM_GPIO)) {
+	if (CONFIG_IS_ENABLED(DM_GPIO)) {
 		struct pca954x_priv *priv = dev_get_priv(dev);
 		int err;
 
@@ -146,7 +146,7 @@
 
 static int pca954x_remove(struct udevice *dev)
 {
-	if (IS_ENABLED(CONFIG_DM_GPIO)) {
+	if (CONFIG_IS_ENABLED(DM_GPIO)) {
 		struct pca954x_priv *priv = dev_get_priv(dev);
 
 		if (dm_gpio_is_valid(&priv->gpio_mux_reset))