dt-bindings: firmware: imx95-scmi: Allow linux,code for protocol@81

dt-bindings: firmware: imx95-scmi: Allow linux,code for protocol@81

BBM protocol supports a single power button, supported by driver
imx-sm-bbm-key.c. By default this is KEY_POWER, but can also be overwritten
using linux,code. Add a reference to this schema and add linux,code as a
supported property.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>