plic: Change device tree compat string to "sifive,plic-1.0.0"

The PLIC's memory map isn't actually specified by the RISC-V ISA manual,
so this shouldn't have an official-looking compat string.  There have
been extensive discussions about this on the Linux mailing lists

    https://lore.kernel.org/lkml/CAL_JsqLpM=UG_9aL8CquhP8SajVav5in7RJRDk9tDd=QrTdVFw@mail.gmail.com/T/#mdbed7f79ba90c1ef7536ae792e8cf62a1fd4d9a0



Signed-off-by: Palmer Dabbelt <palmer@sifive.com>