xen/arm: Pass the timer "clock-frequency" to DOM0 in make_timer_node()

xen/arm: Pass the timer "clock-frequency" to DOM0 in make_timer_node()

If the DT representing the ARM generic timer mentions a clock-frequency,
propragate it to the DT that is built for DOM0.

This is necessary as a workaround for boards (Odroid-XU) where CNTFRQ is not
set or returns a wrong value.

Ideally CNTFRQ should be set by the boot loader. The bootloader should respect
the ARM ARM (see B.8.1.1):
    "The CNTFRQ register is UNKNOWN at reset, and therefore the counter
    frequency must written to CNTFRQ as part of the system boot process."

For the Odroid-XU the SPL BL2 code is entered in NS HYP mode which prevents
the execution of the mcr call to set CNTFRQ.

Signed-off-by: Suriyan Ramasami <suriyan.r@gmail.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>