rtc: sun6i: Add default clock name for LOSC

The RTC's main clock, used internally and exported to the rest of the
SoC, is called "LOSC" (low speed oscillator) through the hardware
documentation.

This patch adds a default name for this clock, in case the device tree
does not provide one. This shouldn't happen, but lets play it safe.

Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>