spi: spi-fsl-dspi: Demistify magic value in SPI_SR_CLEAR

This patch adds the field definitions for the SPI_SR register. The SPI
status register is write-1-to-clear and this value is written at init
time.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20190818180115.31114-5-olteanv@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>