pinctrl: intel: merrifield: Add missed header

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:5...