drivers: base: add description for .coredump() callback

Commit 3c47d19f ("drivers: base: add coredump driver ops") added
a new callback in struct device_driver, but not a kerneldoc description
so here it is.

Fixes: 3c47d19f ("drivers: base: add coredump driver ops")
Signed-off-by: Arend van Spriel <aspriel@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>