mips: octeon: dts/dtsi: Change UART DT node to use clocks property

We already have a clock driver for MIPS Octeon. This patch changes the
Octeon DT nodes to supply the clock property via the clock driver
instead of using an hard-coded value, which is not correct in all cases.

Signed-off-by: Aaron Williams <awilliams@marvell.com>
Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Chandrakala Chavva <cchavva@marvell.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>