arm: Move io_init after vm initialization

To create shared pages, the NS_SHARED bit must be written into the
idmap. Before VM initializations, idmap hasn't necessarily been created.
To write shared pages, access must be done on a IPA with the NS_SHARED
bit. When the stage-1 MMU is enabled, that bit is set in the PTE. But
when the stage-1 MMU is disabled, then the realm must write to the IPA
with NS_SHARED directly.

To avoid changing the whole virtio infrastructure to support pre-MMU in
a realm, move the IO initialization after MMU enablement.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Joey Gouly <joey.gouly@arm.com>