gpiolib: Add WM8958 support to the WM8994 driver

Several of the GPIOs on the WM8994 are fixed function on the WM8958 so
error out if the user tries to request them with gpiolib.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>