dt-bindings: pinctrl: document polarfire soc mssio pin controller

dt-bindings: pinctrl: document polarfire soc mssio pin controller

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.

Document it, including several custom configuration options that stem
from MSS Configurator options (the MSS Configurator is part of the FPGA
tooling for this device). "ibufmd" unfortunately is not a 1:1 mapping
with an MSS Configurator option, unlike clamp-diode or lockdown, and I
do not know the effect of any bits in the field. I have no been able to
find an explanation for these bits in documentation.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Linus Walleij <linusw@kernel.org>