add qemu_fdt_setprop_strings() and use it in most places

[Ben Dooks]

Add a helper for qemu_fdt_setprop_strings() to take a set of strings
to put into a device-tree, which removes several open-coded methods
such as setting an char arr[] = {..} or setting char val[] = "str\0str2";

This is for hw/arm, hw/mips and hw/riscv as well as a couple of cores. It
is not fully tested over all of those, I may not have caught all places
this is to be replaced.