gpio: regmap: Add missing header(s)

Do not imply that some of the generic headers may be always included.
Instead, include explicitly what we are direct user of.

While at it, split out the GPIO group of headers.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/gpio/gpio-regmap.c

@@ -5,11 +5,17 @@
  * Copyright 2020 Michael Walle <michael@walle.cc>
  */
 
-#include <linux/gpio/driver.h>
-#include <linux/gpio/regmap.h>
-#include <linux/kernel.h>
+#include <linux/bits.h>
+#include <linux/device.h>
+#include <linux/err.h>
+#include <linux/io.h>
 #include <linux/module.h>
 #include <linux/regmap.h>
+#include <linux/slab.h>
+#include <linux/types.h>
+
+#include <linux/gpio/driver.h>
+#include <linux/gpio/regmap.h>
 
 struct gpio_regmap {
 	struct device *parent;