MFC r359924-r359925, r359927, r359932, r359965

r359924:
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.

Differential Revision: https://reviews.freebsd.org/D23267

r359925:
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.

Differential Revision: https://reviews.freebsd.org/D23268

r359927:
arm: dwmmc: Use mmc_fdt_helpers

Use the mmc_fdt_parse function instead of parsing everything in the
driver.

r359932:
files: Add mmc_fdt_helpers for mmccam enabled config

X-MFC-With: r359924

r359965:
mmc_fdt_helpers: Do not schedule a card detection is there is no cd gpio

If the fdt node doesn't have a cd-gpios properties or if the node is set
as non-removable we do not init the card detection timeout task as it is
useless so don't schedule it too.

X-MFC-With: r359924