sdhci_fsl_fdt.c: Read supported voltages from dts.

We shouldn't overwrite capability register. Instead, voltages supported
by the controller have to be read from dts, as the hardware doesn't
report correct values.

Submitted by: Lukasz Hajec <lha@semihalf.com>
Reviewed by: manu
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D30123