RISC-V: virt: This is a "sifive,test1" test finisher

The test finisher implements the reset command, which means it's a
"sifive,test1" device.  This is a backwards compatible change, so it's
also a "sifive,test0" device.  I copied the odd idiom for adding a
two-string compatible field from the ARM virt board.

Fixes: 9a2551ed ("riscv: sifive_test: Add reset functionality")
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>