rtc: Add Cadence RTC driver

This patch adds a driver for Cadence RTC controller.

It can be enabled with RTC_DRV_CADENCE Kconfig option.
It supports waking system from sleep modes.

Signed-off-by: Jan Kotas <jank@cadence.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>