mmc: omap_hsmmc: do not embed struct mmc in struct omap_hsmmc_plat

The area for struct mmc can be allocated dynamically. It greatly reduces
the size of struct omap_hsmmc_plat. This is useful in cases where the board
level code declares one or two struct omap_hsmmc_plat because it doesn't
use the Driver Model.

This saves around 740 bytes for the am335x_evm SPL.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Tested-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>