drivers: serial: driver init level based on parent node

Boot level based on parent node (PCI or no PCI device). Some platforms the
PCI bus driver depends on ACPI sub system to retrieve platform information
such as interrupt routing information. But ACPI sub system currently
support only post kernel and hence such platforms the UART driver instance
init should be invoked only post kernel in case parent node is PCI.

Signed-off-by: Najumon B.A <najumon.ba@intel.com>