powerpc: mpc8540: Remove macro CONFIG_MPC8540

Replace CONFIG_MPC8540 with ARCH_MPC8540 in Kconfig and clean up existing
macros.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/doc/README.mpc85xxads b/doc/README.mpc85xxads
index ef92739..4a9aeb3 100644
--- a/doc/README.mpc85xxads
+++ b/doc/README.mpc85xxads
@@ -136,7 +136,7 @@
     CONFIG_BOOKE	    BOOKE(e.g. Motorola MPC85xx, AMCC 440, etc)
     CONFIG_E500		    BOOKE e500 family(Motorola)
     CONFIG_MPC85xx	    MPC8540,MPC8560 and their derivatives
-    CONFIG_MPC8540	    MPC8540 specific
+    CONFIG_ARCH_MPC8540	    MPC8540 specific
     CONFIG_MPC8540ADS	    MPC8540ADS board specific
     CONFIG_MPC8560ADS	    MPC8560ADS board specific
     CONFIG_TSEC_ENET	    Use on-chip 10/100/1000 ethernet for networking