We don't know if the firmware running on the raspberry pi is the same firmware
which is running on all bcm283x devices.
Therefore move the console clock initialization into the rpi-common.c board file.
A future commit will use this function to retrieve the miniuart clock from the
raspberry pi firmware.
No functional changes.

Signed-off-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>