Add qcsmptp(4), a driver to share 32-bit values between (co-)processors.

Add qcsmptp(4), a driver to share 32-bit values between (co-)processors.

The inbound path is typically used as interrupt controller, e.g. to handle
handover and ready interrupts when a remoteproc boots.  The outbound path
seems to be used to stop cores, but we don't do that yet.

ok kettenis@