drivers: serial: ns16550: Fixed a bug related to shell failure

Updated boot priority to PRE_KERNEL_1 for all instances
and removed dependency on PCIe. As shell is not working
in a situation where console is using a UART instance
under PCIe and boot priority set to POST_KERNEL.

Signed-off-by: Anisetti Avinash Krishna <anisetti.avinash.krishna@intel.com>