arm64: zynqmp: Remove not documented is-dual property

Remove is-dual not documented property and also update comment about QSPI
sizes to reflect dual configuration as 16MB + 16MB.
Only single configuration is supported now.

Reported-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/876c53b92f99623bae45d5c0c5ae79ee3e24f745.1628239345.git.michal.simek@xilinx.com