drivers/rtc/Kconfig: add hardware dependency to rtc-moxart

The rtc-moxart driver is only useful on MOXA ART systems so it should
depend on ARCH_MOXART as all other MOXA ART drivers already do.  Add
COMPILE_TEST as an alternative to allow for build testing on other
systems.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Jonas Jensen <jonas.jensen@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>