pinctrl: mediatek: extend struct mtk_pin_field_calc to pinctrl-mtk-common-v2.c

This patch adds members sz_reg fixed in struct mtk_pin_field_calc

- The 'fixed' is used to represent the consecutive pins share the same
bits within the same register with the 1st pin so that it can largely
reduce the entry size a bit.

- The 'sz_reg' is used to indicate the range of bits we use in a register
  that may vary by SoC

The above changes make the code more generic and this is useful as there
might be other existing or future chips all use the same logic to access
their register set and then being a little more abstract could help in the
long run.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>