usb: typec: Add firmware documentation for the Intel PMC mux control

Adding documentation that describes how the PMC mux-agent
function is described in the ACPI tables.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20200507150900.12102-4-heikki.krogerus@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>