On x86 builds the absense of <linux/io.h> makes static analyzer and compiler
unhappy which fails to build the driver.

CHECK   drivers/pinctrl/intel/pinctrl-merrifield.c
drivers/pinctrl/intel/pinctrl-merrifield.c:518:17:
  error: undefined identifier 'readl'
drivers/pinctrl/intel/pinctrl-merrifield.c:570:17:
  error: undefined identifier 'readl'
drivers/pinctrl/intel/pinctrl-merrifield.c:575:9:
  error: undefined identifier 'writel'
drivers/pinctrl/intel/pinctrl-merrifield.c:645:17:
  error: undefined identifier 'readl'
  CC      drivers/pinctrl/intel/pinctrl-merrifield.o
drivers/pinctrl/intel/pinctrl-merrifield.c: In function ‘mrfld_pin_dbg_show’:
drivers/pinctrl/intel/pinctrl-merrifield.c:518:10:
  error: implicit declaration of function ‘readl’
  [-Werror=implicit-function-declaration]
  value = readl(bufcfg);
            ^
drivers/pinctrl/intel/pinctrl-merrifield.c: In function ‘mrfld_update_bufcfg’:
drivers/pinctrl/intel/pinctrl-merrifield.c:575:2:
  error: implicit declaration of function ‘writel’
  [-Werror=implicit-function-declaration]
  writel(value, bufcfg);
    ^
cc1: some warnings being treated as errors

Add header to the top of the module.

Fixes: 4e80c8f5 ("pinctrl: intel: Add Intel Merrifield pin controller support")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>