dt-bindings: mmc: renesas,sdhi: Fix dtbs-check warning

Fix dtbs-check warning pinctrl-names:0:'default' was expected
for r8a77470-iwg23s-sbc.dts file.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210817090313.31858-2-biju.das.jz@bp.renesas.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>