clk: clk_octeon: Add simple MIPS Octeon clock driver

This patch adds a simple clock driver for the Marvell Octeon MIPS SoC
family. Its for IO clock rate passing via DT in some of the Octeon
driver, like I2C. So that we don't need to use the non-mainline API
octeon_get_io_clock().

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Lukasz Majewski <lukma@denx.de>