[ARM] orion5x: ts78xx make more bulletproof the RTC load/unload code

Added checks to the platform_device_(register|add) calls so that if
a device failed to load it would then not later be unloaded; also
added the hooks so that it would not try to unload when the RTC
driver support is compiled out.

Signed-off-by: Alexander Clouter <alex@digriz.org.uk>
Signed-off-by: Nicolas Pitre <nico@cam.org>