mmc: omap_hsmmc: use mmc_of_parse to populate mmc_config

Use the mmc_of_parse library function to populate mmc_config instead of
repeating the same code in host controller driver.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>