board/BuR/common: support timer5 for pwm-backlight

in future we support yet another b&r am335x based board, where Timer 5 is
wired to backlight-driver.

So we introduce a new driver-type '2' to setup timer5 instead timer6.

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Reviewed-by: Simon Glass <sjg@chromium.org>