ARM: sun4i: dt: Add pin muxing options for SPI

These are based on the available SPI configurations of Cubieboard,
Olimex LIME, and PcDuino. There is no pin group for SPI3, as all the
boards seem to use those pins for EMAC.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>