FdtBusDxe: Don't make assumptions about RW access to DT passed by HOB

[andreiw]

gDeviceTreeBase is initialized based on the pointer in DT HOB. This is fine, since the bus driver only does read access. However, if a setprop-like interface is ever added (matching FdtClientDxe functionality), then this will need to change. Instead, fdt_open_into should be used with an appropriately-sized buffer.