dt-bindings: riscv: add scalar crypto ISA extensions description

Add description for scalar crypto ISA extensions:

- Zbkb
- Zbkc
- Zbkx
- Zknd
- Zkne
- Zknh
- Zkr
- Zksed
- Zksh
- Zkt

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231114141256.126749-6-cleger@rivosinc.com


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