x86: tangier: pinmux: add API to configure protected pins

This API is going to be used to configure some pins that are protected
for simple modification.

It's not a comprehensive pinctrl driver but can be turned into one
when we need this in the future. Now it is planned to be used only
in one place. So that's why I decided not to pollute the codebase with a
full-blown pinctrl-merrifield nobody will use.

This driver reads corresponding fields in DT and configures pins
accordingly.

The "protected" flag is used to distinguish configuration of SCU-owned
pins from the ordinary ones.

The code has been adapted from Linux work done by Andy Shevchenko
in pinctrl-merrfifield.c

Signed-off-by: Georgii Staroselskii <georgii.staroselskii@emlid.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: fix build warning]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>