cbus: retu-rtc: move to threaded irq

Move to the generic threaded irq infrastructure
and drop all the retu-specific magic.

Unfortunately, due to the conversion and lack of
docs, retu_rtc_ioctl() had to be dropped, someone
else with access to docs is free to implement it
later considering the new style of the driver.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>