firmware: vpd: Probe via coreboot bus

Remove the ad-hoc coreboot table search. Now the driver will only be
probed when the necessary coreboot table entry has already been found.
Furthermore, since the coreboot bus takes care of creating the device, a
separate platform device is no longer needed.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>