pinctrl: add polarfire soc mssio pinctrl driver

pinctrl: add polarfire soc mssio pinctrl driver

On Polarfire SoC, the Bank 2 and Bank 4 IOs connected to the
Multiprocessor Subsystem (MSS) are controlled by IOMUX_CRs 1 through 6,
which determine what function in routed to them, and
MSSIO_BANK#_IO_CFG_CRs, which determine the configuration of each pin.

Add a driver for this pin controller, including several custom
properties that reflect aspects of the MSS's configuration.

Reuse the Kconfig option for iomux0, since controlling MSSIOs without
iomux0 routing a function to the MSSIOs in question is pointless, and
routing a function to the MSSIOs is equally unhelpful if none of them
are configured to make use of that function.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>