MIPS: Octeon: Add octeon_get_io_clock_rate() for cn63xx

Starting with cn63xx Octeon I/O blocks are clocked at a different rate
than the CPU.  Add a new function octeon_get_io_clock_rate() that
yields the I/O clock rate.

Also rearrange octeon_get_clock_rate() to get the value from the saved
sysinfo structure.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Patchwork: http://patchwork.linux-mips.org/patch/1671/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>