spi: sun4i: Access registers and bits via enum offsets

Allwinner support two different SPI controllers one for A10 and
another for A31 with minimal changes in register offsets and
respective register bits, but the logic for accessing the SPI
master via SPI slave remains nearly similar.

Add enum offsets for register set and register bits, so-that
it can access both classes of SPI controllers.

Assign same control register for global, transfer and fifo control
registers to make the same code compatible with A31 SPI controller.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Stefan Mavrodiev <stefan@olimex.com> # A20-SOM204