MIPS: qi_lb60: Probe RTC driver from DT and use it as power controller

Since we already have a devicetree node for the jz4740-rtc driver, we
don't have to probe it from platform code.

Besides, using the jz4740-rtc driver as the power controller for the
qi_lb60 platform allows us to remove the jz4740 platform power-off code,
since this is the only jz4740-based board upstream.

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>