spi: sun6i: Support Allwinner H3 SPI controller

H3 has two SPI controllers. The size of the buffer is 64 * 8.
(8 bit transfer by 64 entry FIFO)
A31 has four controllers. The size of the buffer is 128 * 8.
(8 bit transfer by 128 entry FIFO)

Register maps are sharable, so sun6i SPI driver is reusable with
device configuration.

Use the variable, 'fifo_depth' instead of fixed value to support both SPI
controllers.

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