iommu/mediatek: Add device link for smi-common and m4u

In the lastest SoC, M4U has its special power domain. thus, If the engine
begin to work, it should help enable the power for M4U firstly.
Currently if the engine work, it always enable the power/clocks for
smi-larbs/smi-common. This patch adds device_link for smi-common and M4U.
then, if smi-common power is enabled, the M4U power also is powered on
automatically.

Normally M4U connect with several smi-larbs and their smi-common always
are the same, In this patch it get smi-common dev from the last smi-larb
device, then add the device_link.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Link: https://lore.kernel.org/r/20210111111914.22211-19-yong.wu@mediatek.com
Signed-off-by: Will Deacon <will@kernel.org>