rtc: armada38x: depend on OF

The driver will compile with warning without OF because armada38x_data and
armada8k_data will be defined but not used. It would be possible to move
then in the #ifdef CONFIG_OF section but then their members will be defined
but not used. Instead of moving most of the driver in the #ifdef, simply
depend on OF.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210202112219.3610853-3-alexandre.belloni@bootlin.com