mmc: omap: Allow OMAP_HSMMC[23]_BASE to be unset

Not all omap families define OMAP_HSMMC[23]_BASE so condition those
cases in omap_mmc_init().

Cc: Sandeep Paulraj <s-paulraj@ti.com>
Signed-off-by: Tom Rini <trini@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>