dt-bindings: mmc: sdhci-pxa: Add minItems to pinctrl-names

Some older boards only require a default pinctrl. Add a minItems
property so these don't cause dt-validate warnings.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202509030625.PBgLIAwG-lkp@intel.com/


Signed-off-by: Duje Mihanović <duje@dujemihanovic.xyz>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>