clocksource: bcm2835: use clock-frequency property when available
If the clock frequency is given by a DT property, use this and don't
try to use the clock handle.
Signed-off-by: Enrico Joerns <ejo@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>