arm64: zynqmp: Add memory reserved node for k26 Kria SOM board

PMUFW (Power Management Unit firmware) requires top 1MB of the lower DDR
memory reserved for its operation, this is missing in k26 SM static dts
file. Bootloader or kernel shouldn't access this location.

Signed-off-by: Sharath Kumar Dasari <sharath.kumar.dasari@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/8ddc7fb4bf40faead7a015ad7e095f3f7d2ec191.1686555698.git.michal.simek@amd.com