dt-bindings: mmc: renesas,sdhi: Add mux-states property

dt-bindings: mmc: renesas,sdhi: Add mux-states property

Add mux controller support for data or control lines that are muxed
between a host and multiple cards.

There are several devices supporting a choice of eMMC or SD on a single
board by both dip switch and gpio, e.g. Renesas RZ/G2L SMARC SoM and
SolidRun RZ/G2L SoM.

In-tree dts for the Renesas boards currently rely on preprocessor macros
and gpio hogs to describe the respective cards.

By adding mux-states property to sdhi controller description, boards can
correctly describe the mux that already exists in hardware - and drivers
can coordinate between mux selection and probing for cards.

Acked-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Josua Mayer <josua@solid-run.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>