drivers: misc: gsc: add support for fan controller

Add support for Gateworks System Controller fan tach input.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
(cherry picked from commit 795a7425a7)

drivers/misc/gsc.c

@@ -330,6 +330,9 @@ static int gsc_hwmon(struct udevice *dev)
 
 			printf("%-8s: %d.%03dV\n", label, val / 1000, val % 1000);
 			break;
+		case 4: /* revolutions per minute */
+			printf("%-8s: %drpm\n", label, val * 30);
+			break;
 		}
 	}