MAINTAINERS: Add Steven and Alyssa as panfrost reviewers

Add Steven Price and Alyssa Rosenzweig as reviewers as they have been the
primary reviewers already.

Cc: Steven Price <steven.price@arm.com>
Cc: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Steven Price <steven.price@arm.com>
Acked-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190823013357.932-1-robh@kernel.org