mmc: introduce mmc_power_init

In device tree, there is vmmc-supply property for SD/MMC.
Introduce mmc_power_init function to handle vmmc-supply.

mmc_power_init will first invoke board_mmc_power_init to
avoid break boards which already implement board_mmc_power_init.

If DM_MMC and DM_REGULATOR is defined, the regulator
will be enabled to power up the device.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>