mmc: core: add helpers mmc_regulator_enable/disable_vqmmc

There's a number of drivers (e.g. dw_mmc, meson-gx, mmci, sunxi) using
the same mechanism and a private flag vqmmc_enabled to deal with
enabling/disabling the vqmmc regulator.

Move this to the core and create new helpers mmc_regulator_enable_vqmmc
and mmc_regulator_disable_vqmmc.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lore.kernel.org/r/71586432-360f-9b92-17f6-b05a8a971bc2@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>