spi: omap3_spi: add am43xx support to omap3_spi

Add support for AM43XX to the omap3_spi driver.

Cc: Jagan Teki <jteki@openedev.com>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Reviewed-by: Jagan Teki <jteki@openedev.com>