hw/board: promote fdt from ARM VirtMachineState to MachineState

hw/board: promote fdt from ARM VirtMachineState to MachineState

The use of FDT's is quite common across our various platforms. To
allow the guest loader to tweak it we need to make it available in
the generic state. This creates the field and migrates the initial
user to use the generic field. Other boards will be updated in later
patches.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210303173642.3805-2-alex.bennee@linaro.org>