gpiolib: Document new gpio_chip.init_valid_mask field

A new field init_valid_mask was added to struct gpio_chip, but it was
not documented.

Fixes: f8ec92a9 ("gpiolib: Add init_valid_mask exported function")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20190701142650.25122-1-geert+renesas@glider.beSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/gpio/driver.h

@@ -194,6 +194,8 @@ struct gpio_irq_chip {
  * @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