ARM: dts: sun7i: add /omit-if-no-ref/ tags to pin group nodes

Since only one alternative at a time is used, and some functions may not
be used at all, this cuts down the size of the board dtb files a bit.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>