Yes, just do:
git checkout master
git pull
git checkout mobiledevicesupport
git merge master
to keep mobiledevicesupport in sync with master.
Then, when you’re ready to put mobiledevicesupport into master, first, merge in master like above, then:
git checkout master
git merge mobiledevicesupport
git push origin master
and that’s it.
The assumption here is that mobilexxx is a topic branch with work that isn’t ready to go into your main branch yet. So only merge into master when mobiledevicesupport is in a good place.