powerpc: MPC8641HPCN: Remove macro CONFIG_MPC8641HPCN

Use TARGET_MPC8641HPCN from Kconfig instead.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/board/freescale/common/pixis.h b/board/freescale/common/pixis.h
index bb0e65e..e6e0f66 100644
--- a/board/freescale/common/pixis.h
+++ b/board/freescale/common/pixis.h
@@ -132,7 +132,7 @@
 	u8 res4[33];
 } __attribute__ ((packed)) pixis_t;
 
-#elif defined(CONFIG_MPC8641HPCN)
+#elif defined(CONFIG_TARGET_MPC8641HPCN)
 typedef struct pixis {
 	u8 id;
 	u8 ver;