Makefile: add u-boot-spl.srec target

The u-boot-spl.srec is needed for some platforms, add target to generate this file.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Tom Rini <trini@konsulko.com>