dt-bindings: crypto: inside-secure,safexcel: add compatible for MT7981

dt-bindings: crypto: inside-secure,safexcel: add compatible for MT7981

The MT7981 as well as the MT7986 have a built-in EIP-97 crypto accelerator.
This commit adds a compatible string for MT7981.

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>