c6x: make dsk6455 the default config

C6X had no defconfig, so DSK6455 is as good as any.
Signed-off-by: NMark Salter <msalter@redhat.com>

arch/c6x/Makefile

@@ -6,6 +6,8 @@
 # for more details.
 #
 
+KBUILD_DEFCONFIG := dsk6455_defconfig
+
 cflags-y += -mno-dsbt -msdata=none
 
 cflags-$(CONFIG_C6X_BIG_KERNEL) += -mlong-calls