firmware: coreboot: Expose the coreboot table as a bus

This simplifies creating device drivers for hardware or information
described in the coreboot table. It also avoids needing to search
through the table every time a driver is loaded.

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