ARM: bcm2708: move to new clock manager and driver for aux-uart

Move to new aux-uart driver (from upstream) and enable it correctly
in the device tree

Note that this may also solve the "serial not 115200 baud"
issue on the rpi3...

With an extension to the driver (requesting clock change
notifications) the driver could change the baud divider when
such a clock change occurs...

It still would require some communication between the firmware
and the kernel, so that this would get propagated.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>