dt-bindings: riscv: add Zimop ISA extension description

Add description for the Zimop (May-Be-Operations) ISA extension which
was ratified in commit 58220614a5f of the riscv-isa-manual.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240619113529.676940-2-cleger@rivosinc.com


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