rtc: Kconfig: Name RK818 in Kconfig for RTC_DRV_RK808

The RK808 and RK818 PMICs are using a similar register map.
We can reuse the rtc driver for the RK818 PMIC. So let's add
the RK818 in the Kconfig description.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>