mmc: host: omap_hsmmc: use mmc_regulator_get_supply() to get regulators

In preparation for using the generic mmc binding for io regulator
("vqmmc"), use mmc_regulator_get_supply() to get vmmc and vqmmc regulators.
Only if "vqmmc" regulator isn't found, fallback to use "vmmc_aux"
regulator.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>