mmc_fdt_helper: Add mmc_fdt_set_power

This helper can be used to enable/disable the regulator and starting
the power sequence of sd/sdio/eMMC cards.

Sponsored by: Diablotin Systems
Differential Revision: https://reviews.freebsd.org/D30291

(cherry picked from commit 03d4e8bb6592fefab7b17f1d163adba4e35a12c2)