Blackfin: document SPI CS limitations with CPHA=0

With the Blackfin on-chip SPI peripheral, there is some logic tied to
the CPHA bit whether the Slave Select Line is controlled by hardware
(CPHA=0) or controlled by software (CPHA=1).  However, the Linux SPI
bus driver assumes that the Slave Select being asserted during the
entire SPI transfer.  So explain these small details for people who
need certain SPI modes with standard CS behavior.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>