doc: Add missing ABNF macros for child num

Commit 5871ff01
added support for generated DT macros to get the number
of child nodes of a node, but we forgot to update
the documentation of the augmented Backus–Naur form
of the DT macros to reflect this addition.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>