rtc: armada38x: Add support for Armada 7K/8K

The Armada 7K/8K use the same RTC IP than the Armada 38x. However the SOC
integration differs in 2 points:
 - MBUS bridge timing initialization
 - IRQ configuration at SoC level

Moreover the Armada 7K/8K have an issue preventing to get the interrupt
from alarm 1. This commit allows to use alarm 2 for these A7K/8K but to
still use alarm 1 for the Armada 38x.

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