A couple of new 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() and qemu_fdt_setprop_strings()
and then applied them to the hw/riscv/sifive_u.c machine.

I thought I should get a review in before I continued on looking
at more possible helpers to make the dtb creation code leaner.