hw/ppc/e500: fix bus-frequency property hardcoded to zero in CPU FDT node

The bus-frequency property in the CPU FDT node was hardcoded to 0.
This is incorrect - it should reflect the actual platform bus clock
frequency, as firmware and RTOSes use it to derive peripheral clock
rates.

Notably, the RTEMS QorIQ BSP uses bus-frequency to program the MPIC
global timer interval. With bus-frequency=0, the timer interval
overflows to ~85 seconds, preventing any clock interrupts from firing.

Fix by adding a bus_freq field to PPCE500MachineClass and using it in
the FDT generator. Set bus_freq = PLATFORM_CLK_FREQ_HZ (400MHz) for
existing machines, matching the existing clock_freq value.

Signed-off-by: Vivien LEGER <vivien.leger@gmail.com>
Reviewed-by: Bernhard Beschow <shentey@gmail.com>
Message-ID: <20260411154535.1451361-1-vivien.leger@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>