spi: zynq-qspi: Support two chip selects

spi: zynq-qspi: Support two chip selects

The Zynq QSPI controller features 2 CS. When the num-cs DT property
is set to 2, the hardware will be initialized to support having two
devices connected over each CS.

In this case, both CS lines are driven by the state of the U_PAGE
(upper page) bit. When unset, the lower page (CS0) is selected,
otherwise it is the upper page (CS1).

Change tested on a custom design featuring two SPI-NORs with different
CS on the Zynq-7000 QSPI bus.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20191108140744.1734-8-miquel.raynal@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>