ppc/pnv.c: fix "system-id" FDT when -uuid is set

ppc/pnv.c: fix "system-id" FDT when -uuid is set

Setting -uuid in the pnv machine does not work:

./qemu-system-ppc64 -machine powernv8,accel=tcg  -uuid 7ff61ca1-a4a0-4bc1-944c-abd114a35e80
qemu-system-ppc64: error creating device tree: (fdt_property_string(fdt, "system-id", buf)): FDT_ERR_BADSTATE

This happens because we're using fdt_property_string(), which is a
sequential write function that is supposed to be used when we're
building a new FDT, in a case where read/writing into an existing FDT.

Fix it by using fdt_setprop_string() instead.

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20211207094858.744386-1-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>