drm/rockchip: Allow driver to be shutdown on reboot/kexec

Leaving the DRM driver enabled on reboot or kexec has the annoying
effect of leaving the display generating transactions whilst the
IOMMU has been shut down.

In turn, the IOMMU driver (which shares its interrupt line with
the VOP) starts warning either on shutdown or when entering the
secondary kernel in the kexec case (nothing is expected on that
front).

A cheap way of ensuring that things are nicely shut down is to
register a shutdown callback in the platform driver.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Tested-by: Vicente Bergas <vicencb@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180805124807.18169-1-marc.zyngier@arm.com
Cc: stable@vger.kernel.org