spi/bcm63xx: make spi subsystem aware of message size limits

The bcm63xx SPI controller does not allow manual control of the CS
lines and will toggle it automatically before and after sending data,
so we are limited to messages that fit in the FIFO buffer. Since the CS
lines aren't available as GPIOs either, we will need to make slave
drivers aware of this limitation so they can handle them accordingly.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>