rtc: jz4740: make the driver buildable as a module again

By using kernel_halt() instead of machine_halt(), we can make the driver
build as a module.
However, jz4740 platforms not loading this module will not be able to power
off.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

Revert "rtc: jz4740: make the driver builtin only"

This reverts commit b9168c53.