docs: ioctl-number.rst: reserve IIO subsystem ioctl() space

docs: ioctl-number.rst: reserve IIO subsystem ioctl() space

Currently, only the 'i' 0x90 ioctl() actually exists and is defined in
'include/uapi/linux/iio/events.h'.

It's the IIO_GET_EVENT_FD_IOCTL, which is used to retrieve and FD for
reading events from an IIO device.
We will want to add more ioct() numbers, so with this change the 'i'
0x90-0x9F space is reserved for IIO ioctl() calls.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210215104043.91251-8-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>