arm: doc: cleanup documentation references to iWMMXT extensions

arm: doc: cleanup documentation references to iWMMXT extensions

Now that the iwmmxt extensions have been removed, clean up the
references to it in the documentation.  We keep the
-mcpu/-mtune/-march references as these are still accepted by the
driver.

gcc/ChangeLog:

	* doc/extend.texi: Remove the iwmmxt intrinsics.
	* doc/md.texi: Remove the iwmmxt-related constraints.