dt-bindings: dsp: fsl,dsp: document 'access-controllers' property

dt-bindings: dsp: fsl,dsp: document 'access-controllers' property

Some DSP instances may have their access to certain peripherals
conditioned by a bus access controller such as the one from the
AIPSTZ bridge.

Add the optional 'access-controllers' property, which may be used
in such cases.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>