pinctrl: cherryview: Convert chv_writel() to use chv_padreg()

chv_writel() is now solely used for cases where we write data
to the PAD registers. In order to simplify callers, calculate
register address inside chv_writel().

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