dt-bindings: riscv: document vector crypto requirements

dt-bindings: riscv: document vector crypto requirements

The Unpriv spec states:
| The Zvknhb and Zvbc Vector Crypto Extensions --and accordingly the
| composite extensions Zvkn, Zvknc, Zvkng, and Zvksc-- require a Zve64x
| base, or application ("V") base Vector Extension. All of the other
| Vector Crypto Extensions can be built on any embedded (Zve*) or
| application ("V") base Vector Extension.

Enforce the minimum requirement via schema.

Link: https://github.com/riscv/riscv-isa-manual/blob/main/src/vector-crypto.adoc#extensions-overview
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250312-flask-relay-b36ee622b2c8@spud
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>