xtensa: Switch to using CONFIG_XTENSA for building device trees

The only use of CONFIG_XTFPGA was to build all of the in-tree device
trees.  Switch to using CONFIG_XTENSA instead of a non-Kconfig symbol.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/xtensa/dts/Makefile b/arch/xtensa/dts/Makefile
index fbbdefa..c22c50a 100644
--- a/arch/xtensa/dts/Makefile
+++ b/arch/xtensa/dts/Makefile
@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0+
 
-dtb-$(CONFIG_XTFPGA) += ml605.dtb ml605_nommu.dtb kc705.dtb kc705_nommu.dtb
+dtb-$(CONFIG_XTENSA) += ml605.dtb ml605_nommu.dtb kc705.dtb kc705_nommu.dtb
 
 include $(srctree)/scripts/Makefile.dts