rtc: optee: add RTC driver for OP-TEE RTC PTA

This drivers allows to communicate with a RTC PTA handled by OP-TEE [1].
This PTA allows to query RTC information, set/get time and set/get
offset depending on the supported features.

[1] https://github.com/OP-TEE/optee_os/pull/5179

Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220308133505.471601-1-clement.leger@bootlin.com