greybus: gpb: Create a "GP Bridge" kernel module

This bundles together the existing GP Bridged PHY protocols that were
part of the Greybus core: USB, UART, SDIO, PWM, and GPIO.  This is now a
stand-alone kernel module.  More logic will be moving here in the future
to handle bridged devices.

Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>
Reviewed-by: Alex Elder <elder@linaro.org>