greybus: arpc: move arpc definitions to their own header

Move the ARPC definitions to their own header.

ARPC is not part of greybus, but is rather an implementation-specific
means of communicating with a certain class of host-device hardware.

Note that the same is true for the APBA USB vendor requests, but we keep
them in the greybus header for the time being.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>