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/at91_gpio.c b/drivers/gpio/at91_gpio.c
index dbfed72..5ea3e77 100644
--- a/drivers/gpio/at91_gpio.c
+++ b/drivers/gpio/at91_gpio.c
@@ -210,7 +210,7 @@
 	return 0;
 }
 
-#ifdef CONFIG_DM_GPIO
+#if CONFIG_IS_ENABLED(DM_GPIO)
 static bool at91_get_port_output(struct at91_port *at91_port, int offset)
 {
 	u32 mask, val;
@@ -457,7 +457,7 @@
 	return 0;
 }
 
-#ifndef CONFIG_DM_GPIO
+#if !CONFIG_IS_ENABLED(DM_GPIO)
 /* Common GPIO API */
 
 int gpio_request(unsigned gpio, const char *label)
@@ -499,7 +499,7 @@
 }
 #endif
 
-#ifdef CONFIG_DM_GPIO
+#if CONFIG_IS_ENABLED(DM_GPIO)
 
 struct at91_port_priv {
 	struct at91_port *regs;