spi: Rename sun4i_spi.c into spi-sunxi.c

Now the same SPI controller driver is reusable in all Allwinner
SoC variants, so rename the existing sun4i_spi.c into spi-sunxi.c
which eventually look like a common sunxi driver.

Also update the function, variable, structure names in driver from
sun4i into sunxi.

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