spl: rename config item SPL_ATF_SUPPORT to SPL_ATF

Having CONFIG_SPL_ATF seems more natural.
Rename it, while it it is easy and there's few boards that use it
(only RK3399 and RK3368 boards).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>