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/net/designware.h b/drivers/net/designware.h
index dea12b7..3519a41 100644
--- a/drivers/net/designware.h
+++ b/drivers/net/designware.h
@@ -7,7 +7,7 @@
 #ifndef _DW_ETH_H
 #define _DW_ETH_H
 
-#ifdef CONFIG_DM_GPIO
+#if CONFIG_IS_ENABLED(DM_GPIO)
 #include <asm-generic/gpio.h>
 #endif
 
@@ -235,7 +235,7 @@
 #ifndef CONFIG_DM_ETH
 	struct eth_device *dev;
 #endif
-#ifdef CONFIG_DM_GPIO
+#if CONFIG_IS_ENABLED(DM_GPIO)
 	struct gpio_desc reset_gpio;
 #endif
 #ifdef CONFIG_CLK