rtc: armada38x: Prepare driver to manage different versions

In order to prepare the introduction of the A7K/A8K version of the RTC,
this commit introduces a new data structure. This structure allows to
handle the differences between the integration of the RTC IP in the
SoCs. It will be:
 - MBUS bridge timing initialization
 - IRQ configuration at SoC level

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>