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>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index c62b82fd..3169bbb 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -702,7 +702,7 @@
 	  means of transmitting U-Boot over a serial line for using in SPL,
 	  with a checksum to ensure correctness.
 
-config SPL_ATF_SUPPORT
+config SPL_ATF
 	bool "Support ARM Trusted Firmware"
 	depends on ARM64
 	help