sdhci: Refactor the generic FDT driver

This patch refactors the 'sdhci_fdt.c' driver by moving all vendor
specific routines into separate files and making the base 'sdhci_fdt'
driver subclassable. The goal is to make adding new FDT-based drivers
easier and more maintainable. No functional change intended.

Reviewed by: manu, imp
Differential Revision: https://reviews.freebsd.org/D48527