sdhci_fsl_fdt: Skip vccq reconfiguration without regulator

There is no need to preform any voltage reconfiguration
in case the vccq regulator is not physically attached to the
slot.

Submitted by: Lukasz Hajec <lha@semihalf.com>
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D30355