arm: allwinner: aw_mmc: Use the mmc_fdt_helper

The fdt properties are now parsed via the help of mmc_fdt_helper functions.
This also adds card detection.
Note that on some boards (like the Pine64) card detection is broken due to
a missing resistor on the cd pin.

MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D23268