firmware: audio_render: add support for float samples and non-power-of-two channels

firmware: hdmi: Allow hdmi channel map to be overridden with a gencmd

firmware: audioplus: limit sample rates to ones supported by hardware

firmware: mailbox: Add property to get memory handle from dispmanx resource
See: https://github.com/raspberrypi/firmware/issues/257

firmware: Allow interrupts to be masked from GPU (e.g. when arm is handling them)
See: https://github.com/raspberrypi/firmware/issues/257

firmware: memory reduction of cutdown firmware (saves about 1M)
See: https://github.com/raspberrypi/linux/issues/503