soc: qcom: socinfo: add 'images' sysfs device subnode

Although image information is already available in sysfs,
only root has the ability to write to the select_image node,
a necessary step for reading the CRM, variant and version
info for a specific image. Additionally, multiple clients
cannot safely read the image information without some
caller-side locking to prevent updates of the select_image
node while one of the other files are being read.

Work around this by introducing a new 'images' node that
can be used to dump information about all images without
the need to write to select_image, and make it readable
to non-root.

Change-Id: I71dfda8c7c170b35a66f2f740202a8a79e8b8c0c
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>