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.                                                  

Changes:

v4:
 - fix checkpatch issues
 - remove error checking in hw/core/guest-loader.c
v3:
 - fix return value for the call                                                
 - add better help text                                                         
v2:                                                                             
 - fix node/path in comment