msm: socinfo: Add support for platform_subtype_id

Add sysfs node to socinfo to read the platform subtype
id. This is meant to deprecate the platform_subtype string
sysfs node.

Change-Id: I4bcb6f9cc6af18fd0cf7ea5aeb6da12a124c9ae1
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>