soc: qcom: expose serial_number field via sysfs

socinfo format v0.10 introduced a serial number field, which is
presently only printed at boot, and not made visible via sysfs.
Make it readable from sysfs also.

Change-Id: I523c6cb8829c0dfee31b35913e085cf3996ef21a
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>