gpio: omap_gpio: Add DM_FLAG_PRE_RELOC flag

With DM enabled in SPL, DM_FLAG_PRE_RELOC is required for
the omap_gpio driver to be bound to the gpio devices.

Therefore, add DM_FLAG_PRE_RELOC flag to the omap_gpio driver.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>