dt-bindings: timer: sifive,clint: add legacy riscv compatible

dt-bindings: timer: sifive,clint: add legacy riscv compatible

While "real" hardware might not use the compatible string "riscv,clint0"
it is present in the driver & QEMU uses it for automatically generated
virt machine dtbs. To avoid dt-validate problems with QEMU produced
dtbs, such as the following, add it to the binding.

riscv-virt.dtb: clint@2000000: compatible:0: 'sifive,clint0' is not one of ['sifive,fu540-c000-clint', 'starfive,jh7100-clint', 'canaan,k210-clint']

Reported-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-riscv/20220803170552.GA2250266-robh@kernel.org/
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220823183319.3314940-2-mail@conchuod.ie
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>