spi: convert spi-bfin-v3.c to a multiplatform driver

Spi v3 controller is not only used on Blackfin. So rename it
and use ioread/iowrite api to make it work on other platform.

Signed-off-by: Scott Jiang <scott.jiang.linux@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>