Add driver for serial-connected Mediatek-based GNSS receivers.

These devices typically boot transmitting vendor specific NMEA output
sequences. The serial port bit rate is read from the device tree
"current-speed".

Note that the driver uses the generic GNSS serial implementation and
therefore essentially only manages power abstracted into three power
states: ACTIVE, STANDBY, and OFF.

For mediatek receivers with a main supply and no enable-gpios, this simply
means that the main supply is disabled in STANDBY and OFF (the optional
backup supply is kept enabled while the driver is bound).

Note that the timepulse-support is left unimplemented.
Signed-off-by: Loys Ollivier <lollivier@baylibre.com>
[ johan: rename backup supply ]
Signed-off-by: Johan Hovold <johan@kernel.org>