ARM: rmobile: Build u-boot-spl.srec on Gen2

The u-boot-spl.srec is needed for the minimon update on Gen2 SoCs,
conveniently generate this file if SPL is enabled.

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