CM3: ARM: bcm2708: moved uart0 to use new clock framework

Moved uart0 to use new clock framework.
At the same time removed the fixed clock clk_uart0
and the corresponding references, as the clock is now
read directly from the configured clocks.

If it ever becomes necessary the following can get added
to modify the base clock rate later:
	assigned-clocks = <&clocks BCM2835_CLOCK_UART>;
	assigned-clock-rates = <X>;

Signed-off-by: DigitalDreamtime <clive.messer@digitaldreamtime.co.uk>