rtc: sun6i: Disable the build as a module

commit 3753941475ae6501dcd1e41832bd0e6c35247d6a upstream.

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.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>