dm: mmc: omap_hsmmc: Add pre-reloc flag to the driver

For platforms that don't use device tree in SPL the only
way to mark this driver as 'required by relocation' is
with the DM_FLAG_PRE_RELOC flag. Add this to ensure that
the driver is bound.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>