ARM: kirkwood: of_serial: fix clock gating by removing clock-frequency

When DT support for kirkwood was first introduced, there was no clock
infrastructure.  As a result, we had to manually pass the
clock-frequency to the driver from the device node.

Unfortunately, on kirkwood, with minimal config or all module configs,
clock-frequency breaks booting because of_serial doesn't consume the
gate_clk when clock-frequency is defined.

The end result on kirkwood is that runit gets gated, and then the boot
fails when the kernel tries to write to the serial port.

Fix the issue by removing the clock-frequency parameter from all
kirkwood dts files.

Booted on dreamplug without earlyprintk and successfully logged in via
ttyS0.

Reported-by: Simon Baatz <gmbnomis@gmail.com>
Tested-by: Simon Baatz <gmbnomis@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>