clocksource/drivers/oxnas: Add OX820 compatible

In order to support the Oxford Semiconductor OX820 SoC, add new
compatible string to rps timer driver.
Also add new string in the dt-bindings.

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