gpio: uapi: use 0xB4 as ioctl() major

The previous 'o' is in conflict and not very orderly assigned.
We want to select an ioctl() major that does not conflict with
the existining ones.

Add the new reserved major (0xB4) to Documentation/ioctl/ioctl-number.txt

Fixes: 3c702e99

 ("gpio: add a userspace chardev ABI for GPIOs")
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>