ARM: dts: Specify clocks for UARTs on bcm11351

The frequency property in "snps,dw-apb-uart" entries are no longer
required if the rate of the external clock can be determined using the
clk api (see e302cd93 serial: 8250_dw: add support for clk api).

This patch replaces the frequency property in the UART nodes of
bcm11351.dtsi with references to the relevant clocks following the
common clock binding.

Signed-off-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Reviewed-by: Matt Porter <matt.porter@linaro.org>
Signed-off-by: Christian Daudt <bcm@fixthebug.org>