flexcan: Prefer device tree clock frequency if available.

If our CAN device's device tree node has a clock-frequency property,
then use that value for the can devices clock frequency.  If not, fall
back to asking the platform/mach code for the clock frequency associated
with the flexcan device.

Signed-off-by: Robin Holt <holt@sgi.com>
Acked-by: Wolfgang Grandegger <wg@grandegger.com&gt;,>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Marc Kleine-Budde <mkl@pengutronix.de>,
Cc: U Bhaskar-B22300 <B22300@freescale.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: socketcan-core@lists.berlios.de,
Cc: netdev@vger.kernel.org,
Cc: PPC list <linuxppc-dev@lists.ozlabs.org>
Cc: devicetree-discuss@lists.ozlabs.org
Signed-off-by: David S. Miller <davem@davemloft.net>