rtc: jz4740: Add support for the RTC in the jz4780 SoC

The RTC unit present in the JZ4780 works mostly the same as the one in
the JZ4740. The major difference is that register writes need to be
explicitly enabled, by writing a magic code (0xA55A) to a "write
enable" register before each access.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Maarten ter Huurne <maarten@treewalker.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>