ARM: sun4i: dt: cubieboard: Enable SPI0

Only SPI0 is enabled, as the schematic denotes it as the only SPI bus,
while other pins are reserved for different peripherals.

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