[gpstime] Check only for RMC and remove the talker id

[nliviu]

The test here should strip the talker id GP and use only the RMC part because GNSS/GPS modules can receive data from several talkers:
GA (Galileo), GB (BeiDou), GL (GLONASS), GN (Combination of multiple satellite systems), GP (Global Positioning System), etc.

Example from a u-blox NEO-M8 which provides concurrent reception of up to 3 GNSS (GPS, Galileo, GLONASS, BeiDou):

GNRMC,140124.00,A,44xx.xxxxx,N,026xx.xxxxx,E,0.330,,180425,,,A*67

PS. It would nice if you could extend this component to replace the gps component from esphome which is only arduino.