iommu/iommu-debug: Add secure_attach debugfs file

It can be useful to perform tests on secure domains.  Add support for
this with a "secure_attach" file that will also set the
DOMAIN_ATTR_SECURE_VMID to the value written to the file.

Example usage:

    # cd /sys/kernel/debug/iommu/tests/<device>
    # echo 0xA > secure_attach
    # echo 0x1000,0x1000,0x1000,1 > map
    # echo 0x1000,0x1000 > unmap
    # echo 0 > secure_attach

Change-Id: I0181e360f3e98e0e47a3af8c6adb71c9c4a3c9b5
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>