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/drivers/ddr/fsl/ctrl_regs.c b/drivers/ddr/fsl/ctrl_regs.c
index b2e92a0..32b0967 100644
--- a/drivers/ddr/fsl/ctrl_regs.c
+++ b/drivers/ddr/fsl/ctrl_regs.c
@@ -1831,7 +1831,7 @@
 	unsigned int clk_adjust;	/* Clock adjust */
 	unsigned int ss_en = 0;		/* Source synchronous enable */
 
-#if defined(CONFIG_ARCH_MPC8541) || defined(CONFIG_MPC8555)
+#if defined(CONFIG_ARCH_MPC8541) || defined(CONFIG_ARCH_MPC8555)
 	/* Per FSL Application Note: AN2805 */
 	ss_en = 1;
 #endif