pinctrl: disallow map table entries with NULL dev_name field

pinctrl: disallow map table entries with NULL dev_name field

Hog entries are mapping table entries with .ctrl_dev_name == .dev_name.
All other mapping table entries need .dev_name set so that they will
match some pinctrl_get() call. All extant PIN_MAP*() macros set
.dev_name.

So, there is no reason to allow mapping table entries without .dev_name
set. Update the code and documentation to disallow this.

Signed-off-by: Stephen Warren <[email protected]>
Acked-by: Dong Aisheng <[email protected]>
Signed-off-by: Linus Walleij <[email protected]>