powerpc: wii_defconfig: Enable GPIO-related options

Now that there's a GPIO driver for the Wii, let's enable the following
drivers:

- the GPIO driver itself
- gpio-keys
- gpio-poweroff
- gpio-leds and a few LED triggers

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>