ptp: Document the PTP_CLK_MAGIC ioctl number

Add PTP_CLK_MAGIC to the userspace-api/ioctl/ioctl-number.rst
documentation file.

Fixes: d94ba80e

 ("ptp: Added a brand new class driver for ptp clocks.")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20211024163831.10200-1-rdunlap@infradead.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>