gpio: exar: Use correct property prefix and document bindings

The device-specific property should be prefixed with the vendor name,
not "linux,", as Linus Walleij pointed out. Change this and document the
bindings of this platform device.

We didn't ship the old binding in a release yet. So we can still change
it without breaking an official API.

Fixes: 380b1e2f ("gpio-exar/8250-exar: Make set of exported GPIOs configurable")
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>