* @dbg_show: optional routine to show contents in debugfs; default code
* will be used when this is omitted, but custom code can show extra
* state (such as pullup/pulldown configuration).
+ * @init_valid_mask: optional routine to initialize @valid_mask, to be used if
+ * not all GPIOs are valid.
* @base: identifies the first GPIO number handled by this chip;
* or, if negative during registration, requests dynamic ID allocation.
* DEPRECATION: providing anything non-negative and nailing the base