arm: add device tree support

arm: add device tree support

If compiled with CONFIG_FDT, allow user to specify a device tree file using
the -dtb argument.  If the machine supports it then the dtb will be loaded
into memory and passed to the kernel on boot.

Signed-off-by: Jeremy Kerr <[email protected]>
Signed-off-by: Grant Likely <[email protected]>
[Peter Maydell: Use machine opt rather than global to pass dtb filename]
Signed-off-by: Peter Maydell <[email protected]>