Xilinx: ARM: Port timer to COMMON_CLK

Xilinx: ARM: Port timer to COMMON_CLK

Timer uses common clock framework, which implies the following:
Timer init is done early during boot. To make clocks available through
COMMON_CLK clock init is called from irq_init() which is called with slab up
and before time_init(). Ugly, but works.

CPU frequency scaling messes up the time base. E.g. 'sleep x' does not sleep x
secionds but x scaled up by the factor the CPU is scaled down from its initial
frequency. I cannot find a way to adjust the clocksource/timekeeping to the
changing timer frequency. For this reason the clocksource in unregistered and
re-registered on frequency changes. See the comments in timer.c for more
details.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>