powerpc: MPC8555: Remove macro CONFIG_MPC8555

Replace CONFIG_MPC8555 with ARCH_MPC8555 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 a911ac8..5cbd9f8 100644
--- a/include/keyboard.h
+++ b/include/keyboard.h
@@ -99,7 +99,7 @@
 #endif /* !CONFIG_DM_KEYBOARD */
 
 #if defined(CONFIG_MPC5xxx) || defined(CONFIG_ARCH_MPC8540) || \
-		defined(CONFIG_ARCH_MPC8541) || defined(CONFIG_MPC8555)
+		defined(CONFIG_ARCH_MPC8541) || defined(CONFIG_ARCH_MPC8555)
 int ps2ser_check(void);
 #endif