overlays: audremap: Support GPIOs 18 & 19

PWM audio can also be used on GPIOs 18 and 19, so add the pins_18_19
parameter to select that location. pins_12_13 explicitly chooses GPIOs
12 and 13, although this is the default behaviour so is there only for
completeness.

See: https://github.com/raspberrypi/firmware/issues/1178



Signed-off-by: Phil Elwell <phil@raspberrypi.org>