sdhci_xenon: split driver file into generic file and fdt parts

This patch splits driver code into two seperate files sdhci_xenon.c
and sdhci_xenon_fdt.c. This will allow future implementation of ACPI
discovery of sdhci on Xenon chips.

Reviewed by: mw
Sponsored by: Semihalf
Differential revision: https://reviews.freebsd.org/D31599