Check all compatible strings on uart devices in powerpc

Summary:
Some device trees put "fsl,ns16650" first in the compatible list. This causes
the probe code to choke, even though the device is compatible with ns16650, and
has it listed later in the tree.

Reviewed by: nwhitehorn
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D7676