driver: Create sysfs entries for all active cells

Create a kobject for every existing cell, including the root cell, and
add it to sysfs. This will allow to export state information etc. about
active cells later on.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
driver: Catch duplicate cell creation

Avoid trying to create a cell twice. This will only trigger a memory
access violation when writing to the now reserved memory of the cell,
effectively offlining the Linux CPU that tried it.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>