spi: spi-zynqmp-gqspi: Add two chip select support

ZynqMP GQSPI controller can support up to two chip selects but the current
GQSPI driver only support CS0. With this update and num-cs DT property set
to 2 GQSPI driver can now support two slave devices each connected to one
chip select.

GQSPI driver configures the Lower CS and Upper CS based on the reg DT
property.

Changes tested on ZynqMP board with two SPI-NOR flashes each connected
to a different CS.

Signed-off-by: Amit Kumar Mahapatra <amit.kumar-mahapatra@xilinx.com>
Link: https://lore.kernel.org/r/20220512145820.20425-1-amit.kumar-mahapatra@xilinx.com
Signed-off-by: Mark Brown <broonie@kernel.org>