greybus: gpbridge: rename 'gpbridge' to 'gbphy' everywhere

The 'gpbridge' name didn't relaly reflect what the bus is; which
is a bus for bridged-phy devices. So, rename all instances
of 'gpbridge' to more appropriate 'gbphy'

Testing Done:
Build and boot tested. 'lsgb' will stop displaying 'GPBridge' devices
until I change the library to reflect this change.

Signed-off-by: Sandeep Patil <patil_sandeep@projectara.com>
Suggested-by: Greg Kroah-Hartman <gregkh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>