riscv: dts: microchip: drop duplicated MMC/SDHC node

Devicetree source is a description of hardware and hardware has only one
block @20008000 which can be configured either as eMMC or SDHC.  Having
two node for different modes is an obscure, unusual and confusing way to
configure it.  Instead the board file is supposed to customize the block
to its needs, e.g. to SDHC mode.

This fixes dtbs_check warning:
  arch/riscv/boot/dts/microchip/microchip-mpfs-icicle-kit.dt.yaml: sdhc@20008000: $nodename:0: 'sdhc@20008000' does not match '^mmc(@.*)?$'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>