arm: dts: xilinx: Remove undocumented is-dual property

Xilinx was using in past is-dual property for QSPIs to reflect their
configurations. But handling for them never reached upstream code that's
why better to remove them.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/15980560b98672959a889ff9970cbe9540b4ed69.1695808563.git.michal.simek@amd.com