mmc: am654_sdhci: Allow driver to probe without PDs specified

We would like to use the driver even without power domains being
specified for cases such as during early boot when the required power
domains have already gotten enabled by the SoC's boot ROM and such
explicit initialization is not needed and possible.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>