clocksource/drivers/rda: Add clock driver for RDA8810PL SoC

Add clock driver for RDA Micro RDA8810PL SoC supporting OSTIMER
and HWTIMER.

RDA8810PL has two independent timers: OSTIMER (56 bit) and HWTIMER
(64 bit). Each timer provides optional interrupt support. In this
driver, OSTIMER is used for clockevents and HWTIMER is used for
clocksource.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>