ARM: dts: Fix Makefile target for sun4i-a10-itead-iteaduino-plus

Commit 79ae3e66 (ARM: dts: sun4i: Add Iteaduino Plus A10) added a new
make target for the sun4i-a10-itead-iteaduino-plus dts file, but mistakenly
used .dts instead of the correct .dtb suffix.  This resulted in a build error
like:

        scripts/Makefile.dtbinst:42: target 'sun4i-a10-itead-iteaduino-plus.dts' doesn't match the target pattern

when doing a make dtbs_install.

Fix it to use the proper file name.

Signed-off-by: Josh Boyer <jwboyer@fedoraproject.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>