spi/pl022: initialize burstsize from FIFO trigger level

Configure the DMA burstsize from the FIFO trigger level supplied
with the controller configuration data. This is based on a patch
from Virupax, but I rewrote it differently.

Reported-by: Virupax Sadashivpetimath <virupax.sadashivpetimath@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>