mmc: switch mmc_helper to device_ api

Add generic mmc_helper which uses newly introduced device_*_property
api. Thanks to this change the sd/mmc drivers will be capable
of parsing both DT and ACPI description.

Ensure backward compatibility for all mmc_fdt_helper users.

Reviewed by: manu, mw
Sponsored by: Semihalf
Differential revision: https://reviews.freebsd.org/D31598