Specify PCI based UART for earlyprintk

Add support for specifying PCI based UARTs for earlyprintk
using a syntax like "earlyprintk=pciserial,00:18.1,115200",
where 00:18.1 is the BDF of a UART device.

[Slightly tidied from Stuart's original patch]
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>