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>

Documentation/userspace-api/ioctl/ioctl-number.rst

@@ -245,6 +245,7 @@ Code  Seq#    Include File                                           Comments
 'i'   00-3F  linux/i2o-dev.h                                         conflict!
 'i'   0B-1F  linux/ipmi.h                                            conflict!
 'i'   80-8F  linux/i8k.h
+'i'   90-9F  `linux/iio/*.h`                                         IIO
 'j'   00-3F  linux/joystick.h
 'k'   00-0F  linux/spi/spidev.h                                      conflict!
 'k'   00-05  video/kyro.h                                            conflict!