dt-bindings: rng: mtk-rng: add SMC-based TRNG variants

dt-bindings: rng: mtk-rng: add SMC-based TRNG variants

Add compatible strings for MediaTek SoCs where the hardware random number
generator is accessed via a vendor-defined Secure Monitor Call (SMC)
rather than direct MMIO register access:

  - mediatek,mt7981-rng
  - mediatek,mt7987-rng
  - mediatek,mt7988-rng

These variants require no reg, clocks, or clock-names properties since
the RNG hardware is managed by ARM Trusted Firmware-A.

Relax the $nodename pattern to also allow 'rng' in addition to the
existing 'rng@...' pattern.

Add a second example showing the minimal SMC variant binding.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>