powerpc: use CONFIG_IS_ENABLED() when checking for DM_SERIAL in include/asm/config.h

Use the CONFIG_IS_ENABLED() macro when checking for DM_SERIAL so that
CFG_SYS_NS16550_CLK is not defined as get_serial_clock() in SPL if SPL
does not have DM_SERIAL enabled.

Signed-off-by: Marek Mojík <marek.mojik@nic.cz>
Reviewed-by: Marek Behún <kabel@kernel.org>
diff --git a/arch/powerpc/include/asm/config.h b/arch/powerpc/include/asm/config.h
index f0702ca..f61f4e1 100644
--- a/arch/powerpc/include/asm/config.h
+++ b/arch/powerpc/include/asm/config.h
@@ -39,7 +39,7 @@
 
 /* The FMAN driver uses the PHYLIB infrastructure */
 
-#if defined(CONFIG_DM_SERIAL) && !defined(CONFIG_CLK_MPC83XX)
+#if CONFIG_IS_ENABLED(DM_SERIAL) && !defined(CONFIG_CLK_MPC83XX)
 /*
  * TODO: Convert this to a clock driver exists that can give us the UART
  * clock here.