dm: sunxi: sata: Don't build sata support into SPL

dm: sunxi: sata: Don't build sata support into SPL

This is not used in SPL so we do not need to compile it. Make this change
before adding driver-model support to the driver, to avoid build errors.
With driver model we define a U_BOOT_DRIVER() which would otherwise be
present in SPL and not be garbage-collected when building.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>