clocksource/drivers/renesas,ostm: Make RENESAS_OSTM symbol visible

As RZ/G2L uses the ARM Architected Timer as system timer, enabling
the OSTM is not mandatory.

Make RENESAS_OSTM symbol visible, so that user can enable it
when needed.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20211112184413.4391-5-biju.das.jz@bp.renesas.com


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>