arm: dt: zynq: Use 'status' property for UART nodes

Set the default status for UARTs to disabled in the zynq-7000.dtsi file
and let board dts files enable the UARTs on demand.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Reviewed-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>