arm64: zynqmp: Describe assigned-clocks for uarts

Describe assigned-clocks for both uarts. SOM is using this functionality.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/bddbb81209a4567b0939c5d2d0ecb42fdfcd71ea.1695036114.git.michal.simek@amd.com