gpio: move descriptors into gpio_device

We need gpio_device to hold the descriptors so that they can
be lifecycled with the struct gpio_device held from userspace.
Move the descriptor array into gpio_device. Also rename it from
"desc" (singularis) to "descs" (pluralis) to reflect the fact
that it is an array.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>