rtc: sh: add support for rza series

This same RTC is used in RZ/A series MPUs, therefore with some slight
changes, this driver can be reused. Additionally, since ARM architectures
require Device Tree configurations, device tree support has been added.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>