Adding severity when reporting an error

Severity is part of the PEL User Header section, and is used
to specify the PEL severity. This commit adds an option to pass
severity parameter while logging PEL. This will help to classify PELs
based on their severity. The commit also maintains uniformity in
the PEL Error logged.

Tested: By creating the error logs(pel) in error scenarios.

Signed-off-by: Pavithra Barithaya <pavithra.b@ibm.com>
Change-Id: Ic084d49b8ffe9aaea2c36c9fefa95a10a9c1c3ec