powerpc: cyrus: Separate P5020/P5040 config options

Instead of using EXTRA options in defconfig, use two targets
in Kconfig to select correct SoC.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/board/varisys/cyrus/Kconfig b/board/varisys/cyrus/Kconfig
index d9ea7ef..c7b5253 100644
--- a/board/varisys/cyrus/Kconfig
+++ b/board/varisys/cyrus/Kconfig
@@ -1,4 +1,4 @@
-if TARGET_CYRUS
+if TARGET_CYRUS_P5020 || TARGET_CYRUS_P5040
 
 config SYS_BOARD
 	default "cyrus"