powerpc/of: added documentation for spi chipselects

Added devicetree binding documentation for gpios used as chipselect. The
code to evaluate these is already present in spi_mpc8xxx.c.

Signed-off-by: Ernst Schwab <eschwab@online.de>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>