ARM: rmobile: Convert TPL to SPL

There is currently no use for building the SPL anymore, since the
SPI loader can easily be replaced by TPL and TPL does load U-Boot
directly. Upgrade TPL to SPL and replace what used to be SPL with
it. This way we build the U-Boot sources only twice, not thrice.

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