arm: dts: sync dts for Action Semi S900

Synchronize device tree bindings with v5.5-rc6 tag with commit id
"b3a987b0264d".

Also, it removes older clock binding defined for S900 along with undocumented
compatible string "actions,s900-serial" from serial driver and adapts clock
driver to cater to new bindings.

Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Amit Singh Tomar <amittomer25@gmail.com>