scripts/dtc: Add missing fdtoverlay to gitignore

commit e515377a59932c43be222a3794d08983d5504ce1 from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git



commit c3476d2f upstream.

Commit 0da6bcd9 ("scripts: dtc: Build fdtoverlay tool") enabled
building fdtoverlay, but failed to add it to .gitignore.

Also add a note to keep hostprogs in sync with .gitignore.

Fixes: b862bdb74f46 ("scripts: dtc: Build fdtoverlay tool")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
[s-anna@ti.com: cherry-pick commit 'c3476d2f' from v5.12]
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>