ARM: renesas: Simplify weak symbols

Simplify the weak symbols use in cpu_info.c by using the __weak macro.
The result is identical, but the syntax is much simpler. Furthermore,
drop three more rmobile* symbols which are now unused.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Paul Barker <paul.barker.ct@bp.renesas.com>