ARM: multi_v7_defconfig: add Panfrost driver

With the goal of making it easier for CI services such as KernelCI to
run tests for it.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Olof Johansson <olof@lixom.net>