ARM: rmobile: Split U-Boot and SPL sources on Porter

Pull the SPL code from porter.c into a separate file in
preparation for the addition of system initialization code.
No functional change.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
diff --git a/board/renesas/porter/Makefile b/board/renesas/porter/Makefile
index b0cfb1b..c237ee5 100644
--- a/board/renesas/porter/Makefile
+++ b/board/renesas/porter/Makefile
@@ -7,4 +7,8 @@
 # SPDX-License-Identifier: GPL-2.0
 #
 
+ifdef CONFIG_SPL_BUILD
+obj-y	:= porter_spl.o
+else
 obj-y	:= porter.o qos.o
+endif