SALTA, Argentina, Sept 14 (Reuters) - In Argentina's remote northern Salta province, the silence of the desert landscape is broken only by the hum of machinery pumping salt-water brine to extract lithium, a sign of accelerating efforts to ride the global electric vehicle boom. Beneath the South American country's highland salt flats, reached by winding mountain roads, is buried the world's third largest reserve of the ultra-light battery metal, which has seen a price spike over the past year on the back of a global push towards greener modes of transport. Already the fourth top producer of lithium worldwide, Argentina's...