PNP: constify driver name

struct pnp_driver has name set as char* instead of const char* like platform_driver, pci_driver, usb_driver, etc...
Let's unify a bit by setting name as const char*.
Furthermore, all users of this structures set name from already const
data.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>