MIPS: Octeon: Use I/O clock rate for calculations.

The I2C and UARTS are clocked by the I/O clock, use its rate for these
devices.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Patchwork: http://patchwork.linux-mips.org/patch/1670/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>