arm64: zynqmp: Used fixed-partitions for QSPI in k26

Using fixed partitions is recommended way how to describe QSPI. Also add
label for qspi flash memory to be able to reference it in future.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/a84f7ce8d6472fce66539ba29d31fbaae511d94b.1655732762.git.michal.simek@amd.com