clocksource/drivers/ostm: Add renesas-ostm timer driver

This patch adds a OSTM driver for the Renesas architecture.
The OS Timer (OSTM) has independent channels that can be
used as a freerun or interval times.
This driver uses the first probed device as a clocksource
and then any additional devices as clock events.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>