spi: fsi: Fix contention in the FSI2SPI engine

There was nothing to protect multiple SPI controllers on the same FSI2SPI
device from being accessed through the FSI2SPI device at the same time.
For example, multiple writes to the command and data registers might occur
for different SPI controllers, resulting in complete chaos in the SPI
engine. To prevent this, add a FSI2SPI device level mutex and lock it in
the SPI register read and write functions.

Fixes: bbb6b2f9

 ("spi: Add FSI-attached SPI controller driver")
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20211026193327.52420-1-eajames@linux.ibm.com
Signed-off-by: Mark Brown <broonie@kernel.org>