riscv: dts: microchip: drop unused pinctrl-names

pinctrl-names without pinctrl-0 does not have any sense:

  arch/riscv/boot/dts/microchip/microchip-mpfs-icicle-kit.dt.yaml: sdhc@20008000: 'pinctrl-0' is a dependency of 'pinctrl-names'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>