rtc: sun6i: Disable the build as a module

Since we have to provide the clock very early on, the RTC driver cannot be
built as a module. Make sure that won't happen.

Cc: stable@vger.kernel.org
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>