dt-bindings: mmc: renesas,sdhi: Group single const value items into an enum list

Group single const value items into an enum list.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240430145937.133643-2-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>