iommu/msm: Use standard driver registration

It's been a long time since there was any reason to register IOMMU
drivers early. Convert to the standard platform driver helper.

CC: Andy Gross <agross@kernel.org>
CC: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/all/05ca5e1b29bdd350f4e20b9ceb031a2c281e23d2.1644005728.git.robin.murphy@arm.com/
Signed-off-by: Joerg Roedel <jroedel@suse.de>