dt-bindings: riscv: add Zacas ISA extension description

Add description for the Zacas ISA extension which was ratified recently.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231220155723.684081-4-cleger@rivosinc.com


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