dt-bindings: mmc: spacemit,sdhci: add pinctrl support for voltage switching

dt-bindings: mmc: spacemit,sdhci: add pinctrl support for voltage switching

Document pinctrl properties to support voltage-dependent pin
configuration switching for UHS-I SD card modes.

Add optional pinctrl-names property with two states:
- "default": For 3.3V operation with standard drive strength
- "state_uhs": For 1.8V operation with optimized drive strength

These pinctrl states allow the SDHCI driver to coordinate voltage
switching with pin configuration changes, ensuring proper signal
integrity during UHS-I mode transitions.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Iker Pedrosa <ikerpedrosam@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>