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/include/configs/at91-sama5_common.h b/include/configs/at91-sama5_common.h
index d19fd31..6e9793a 100644
--- a/include/configs/at91-sama5_common.h
+++ b/include/configs/at91-sama5_common.h
@@ -9,6 +9,8 @@
 #ifndef __AT91_SAMA5_COMMON_H
 #define __AT91_SAMA5_COMMON_H
 
+#include <linux/kconfig.h>
+
 /* ARM asynchronous clock */
 #define CONFIG_SYS_AT91_SLOW_CLOCK      32768
 #define CONFIG_SYS_AT91_MAIN_CLOCK      12000000 /* from 12 MHz crystal */
@@ -18,11 +20,10 @@
 #endif
 
 /* general purpose I/O */
-#ifndef CONFIG_DM_GPIO
+#if !CONFIG_IS_ENABLED(DM_GPIO)
 #define CONFIG_AT91_GPIO
 #endif
 
-
 /*
  * BOOTP options
  */