pinctrl: oxnas: Add support for OX820

Add support for the Oxford Semiconductor OX820 which is similar as OX810 but
has 50 pins and two registers banks to setup alternate functions.
Add specific pins, groups and functions structures.
Add DT match data to select corresponding support.
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>