gnss: add receiver type support

Add a "type" device attribute and a "GNSS_TYPE" uevent variable which
can be used to determine the type of a GNSS receiver. The currently
identified types reflect the protocol(s) supported by a receiver:

	"NMEA"	NMEA 0183
	"SiRF"	SiRF Binary
	"UBX"	UBX

Note that both SiRF and UBX type receivers typically support a subset of
NMEA 0183 with vendor extensions (e.g. to allow switching to the vendor
protocol).

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>