Convert CONFIG_SPL_SERIAL_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/a4m2k_defconfig b/configs/a4m2k_defconfig
index e688efe..3f2a58a 100644
--- a/configs/a4m2k_defconfig
+++ b/configs/a4m2k_defconfig
@@ -1,6 +1,7 @@
 CONFIG_PPC=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_MPC5xxx=y
 CONFIG_TARGET_A3M071=y