pinctrl: starfive: Add pinctrl driver for StarFive SoCs

Add a combined pinctrl and GPIO driver for the JH7100 RISC-V SoC by
StarFive Ltd. This is a test chip for their upcoming JH7110 SoC, which
is said to feature only minor changes to these pinctrl/GPIO parts.

For each "GPIO" there are two registers for configuring the output and
output enable signals which may come from other peripherals. Among these
are two special signals that are constant 0 and constant 1 respectively.
Controlling the GPIOs from software is done by choosing one of these
signals. In other words the same registers are used for both pin muxing
and controlling the GPIOs, which makes it easier to combine the pinctrl
and GPIO driver in one.

I wrote the pinconf and pinmux parts, but the GPIO part of the code is
based on the GPIO driver in the vendor tree written by Huan Feng with
cleanups and fixes by Drew and me.

Datasheet: https://github.com/starfive-tech/JH7100_Docs/blob/main/JH7100%20Data%20Sheet%20V01.01.04-EN%20(4-21-2021).pdf


Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Co-developed-by: Huan Feng <huan.feng@starfivetech.com>
Signed-off-by: Huan Feng <huan.feng@starfivetech.com>
Co-developed-by: Drew Fustini <drew@beagleboard.org>
Signed-off-by: Drew Fustini <drew@beagleboard.org>
Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>