uboot-imx

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>