Add missing ti,itap-del-sel-ddr50 property to schema to clear up the
following warnings.

mmc@4fb0000: Unevaluated properties are not allowed ('ti,itap-del-sel-ddr50' was unexpected)
Signed-off-by: Matt Ranostay <mranostay@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20221013024021.121104-1-mranostay@ti.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>