spi: sun4: Add A31 spi controller support

The usual SPI transmission protocol in Allwinner A10 and A31
controllers share similar context with minimal changes in register
offsets along with few additional register bits on A31.

So, add A31 spi controller support in existing sun4i_spi with A31
specific register offsets and bits.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>