dt-bindings: usb: add documentation for typec port controller(TCPCI)

TCPCI stands for typec port controller interface, its implementation
has full typec port control with power delivery support, it's a
standard i2c slave with GPIO input as irq interface, detail see spec
"Universal Serial Bus Type-C Port Controller Interface Specification
Revision 1.0, Version 1.1"

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Li Jun <jun.li@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>