spi: sun6i: Add binding for Allwinner H3 SPI controller

H3 SPI has same architecture as A31 except FIFO capacity.
To configure the buffer size separately, compatible property should be
different. Optional DMA specifiers and example are added.

Signed-off-by: Milo Kim <woogyom.kim@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>