Those functions are here to help fdt mmc controller drivers to parse
the dts to find the supported speeds and the regulators.
Not all DTS have every settings properly defined so host controller
will still have to add some caps themselves.
It also add a mmc_fdt_gpio_setup function which will read the cd-gpios
property and register it as the CD pin.
If the pin support interrupts one will be registered and the cd_helper
function will be called.
If the pin doesn't support interrupts the internal taskqueue will poll
for change and call the same cd_helper function.
mmc_fdt_gpio_setup will also parse the wp-gpio property and MMC drivers
can know the write-protect pin value by calling the
mmc_fdt_gpio_get_readonly function.

MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D23267