ARM: mvebu: remove clock-frequency of serial port Device Tree nodes

Now that the Armada 370/375/38x/XP SoC-level Device Tree files have
the proper "clocks" property in their UART controllers node, it is no
longer useful to have the clock-frequency property defined in the
board-level Device Tree files.

Therefore, this commit gets rid of all the useless 'clock-frequency'
properties.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Link: https://lkml.kernel.org/r/1397806908-7550-5-git-send-email-thomas.petazzoni@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>