Blackfin: spi-docs: further clarify GPIO CS behavior with various modes

Signed-off-by: Mike Frysinger <[e-mail]>

Documentation/blackfin/bfin-spi-notes.txt

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 In most cases you can utilize SPI MODE_3 instead of MODE_0 to work-around this
 behavior. If your SPI slave device in question requires SPI MODE_0 or MODE_2
 timing, you can utilize the GPIO controlled SPI Slave Select option instead.
+In this case, you should use GPIO based CS for all of your slaves and not just
+the ones using mode 0 or 2 in order to guarantee correct CS toggling behavior.
 
 You can even use the same pin whose peripheral role is a SSEL,
 but use it as a GPIO instead.