gpio: uapi: clarify using v2 rather than v1

The documentation contains notes like
 This struct is part of ABI v1 and is deprecated.
 Use struct gpio_v2_line_info instead.

This could be interpreted to mean the structs can be directly
substituted in v1 calls.  Clarify that the user should use the
corresponding v2 ioctl() and structs.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>