dt-bindings: timer: Add T-HEAD TH1520 clint

Add compatible string for the T-HEAD TH1520 clint.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>