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/include/keyboard.h b/include/keyboard.h
index 6725e48..e26f69e 100644
--- a/include/keyboard.h
+++ b/include/keyboard.h
@@ -98,7 +98,7 @@
 extern void pckbd_leds(unsigned char leds);
 #endif /* !CONFIG_DM_KEYBOARD */
 
-#if defined(CONFIG_MPC5xxx) || defined(CONFIG_MPC8540) || \
+#if defined(CONFIG_MPC5xxx) || defined(CONFIG_ARCH_MPC8540) || \
 		defined(CONFIG_MPC8541) || defined(CONFIG_MPC8555)
 int ps2ser_check(void);
 #endif