[v2] pair of device-tree helpers

[Ben Dooks]

I've been doing a bit of looking at riscv and dt creation, and                  
was thinking the following two helper functions would be useful                 
so implemented qemu_fdt_setprop_reg64_map() and qemu_fdt_setprop_strings()      
    
and then applied them to the hw/riscv/sifive_u.c machine.