rtc: mxc{,_v2}: enable COMPILE_TEST

Extend code coverage for the rtc-mxc and rtc-mxc-v2 drivers.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20201121224529.568237-1-alexandre.belloni@bootlin.com