arm64: zynqmp: Add power domain description for PL

PL has own power domain which is not described in DT. That's why add it
there by default.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/b38e2ea95dab434bc007f9ed6c438c68149744bf.1652262769.git.michal.simek@amd.com