ARM: dts: sun7i: Change 32768 Hz oscillator node name to clk@N style

Device tree naming conventions state that node names should match
the nodes function. Change external low speed oscillator node name
to match.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>