arm: am43xx: enable spi clock

Add spi clock to the list of am43xx basic clocks to make the SPI
subsystem available on am43xx systems.

Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Reviewed-by: Jagan Teki <jteki@openedev.com>