MIPS: jz4740: Remove obsolete code

This commit removes two things:
- The platform_device that corresponds to the RTC driver, since we now
  probe this driver from devicetree;
- The platform power-off code, since all the jz4740-based platforms are
  now using the jz4740-rtc driver as the system power controller.

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>