dt-bindings: riscv: Add Zabha ISA extension description

Add description for the Zabha ISA extension which was ratified in April
2024.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Andrea Parri <parri.andrea@gmail.com>
Link: https://lore.kernel.org/r/20241103145153.105097-5-alexghiti@rivosinc.com


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