clocksource/drivers/oxnas-rps: Add Oxford Semiconductor RPS Dual Timer

Add clocksource and clockevent driver from dual RPS timer.
The HW provides a dual one-shot or periodic 24bit timers,
the drivers set the first one as tick event source and the
second as a continuous scheduler clock source.
The timer can use 1, 16 or 256 as pre-dividers, thus the
clocksource uses 16 by default.

CC: Ma Haijun <mahaijuns@gmail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>