sdhci_fsl_fdt: specify base clk divisor per SoC

Only LS1046A and LS1028A require the base clk to be divided by 2.
Implement that by moving the divider to a SoC specific data.
This commit fixes base clk setup for the entire SoC family,
including the already suported LS2160A.

Submitted by: Lukasz Hajec <lha@semihalf.com>
Reviewed by: manu
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D30120