pinctrl: intel: merrifield: Introduce mrfld_read_bufcfg()

mrfld_read_bufcfg() helper checks if pin is correct and reads back
the current value of corresponding BUFCFG register.

While it adds lines of code it will be easier to maintain in the future.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>