timer: sifive_clint: Support the official clint DT bindings

Linux kernel commit a2770b57d083 ("dt-bindings: timer: Add CLINT bindings")
adds the official DT bindings for CLINT, which uses "sifive,clint0"
as the compatible string. "riscv,clint0" is now legacy and has to
be kept for backward compatibility of legacy systems.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>