MAINTAINERS: Add entries for missing ARM boards

Add entries for the boards "mcimx6ul-evk", "mcimx7d-sabre", "raspi2",
"raspi3", "sabrelite", "vexpress-a15", "vexpress-a9" and "virt".
While we're at it, also adjust the "i.MX31" section a little bit,
so that the wildcards there do not match anymore for unrelated files
(e.g. the new hw/misc/imx6ul_ccm.c file).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1542184999-11145-1-git-send-email-thuth@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>