arm64: zynqmp: Add command for disabling loading other overlays

Add command "zynqmp pmufw node close" to disable permission to load
additional pmufw config overlays. This command will make sure that any
other sw will ask for changing permission.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/cfa5cc7909eb8deb23eb0f11c26954cbaddeb861.1642163135.git.michal.simek@xilinx.com