power: smb135x-charger: Add mutex locking in parallel enable path

Add  locking in the parallel charger present callback to avoid
a race while configuring it from the primary charger.

CRs-Fixed: 1089991
Change-Id: Ice7f234dcf406f12dadb561d46e049efd9d6ad88
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>