firmware: coreboot: Let OF core populate platform device

Now that the /firmware/coreboot node in DT is populated by the core DT
platform code with commit 3aa0582f ("of: platform: populate
/firmware/ node from of_platform_default_populate_init()") we should and
can remove the platform device creation here. Otherwise, the
of_platform_device_create() call will fail, the coreboot of driver won't
be registered, and this driver will never bind. At the same time, we
should move this driver to use MODULE_DEVICE_TABLE so that module
auto-load works properly when the coreboot device is auto-populated and
we should drop the of_node handling that was presumably placed here to
hold a reference to the DT node created during module init that no
longer happens.

Cc: Wei-Ning Huang <wnhuang@chromium.org>
Cc: Julius Werner <jwerner@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Cc: Samuel Holland <samuel@sholland.org>
Reviewed-by: Sudeep Holla <Sudeep.Holla@arm.com>
Fixes: 3aa0582f ("of: platform: populate /firmware/ node from of_platform_default_populate_init()")
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>