drivers: uart: ns16550: Move all PCIe-using nodes to a PCIe bus in DTS

The existing method of testing for any of the first four DT instances
having the pcie property feels a bit clumsy and will get more so when
support for more than four UARTs is added. A much more cleaner way to
do this (and more correct probably as well) is to list any PCIe-based
UART nodes under a pcie bus in the Device Tree hierarchy.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>