greybus: Documentation: Add sysfs information about bridged-phy device

Update the documentation (sysfs-bus-greybus) for recent conversion of
bridged-phy drivers to real drivers.

Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>