power: smb135x-charger: Add additional debugging capability

1. Debugfs node to list the current irq count for the SMB irqs
2. Debugfs node to prevent reads from the SMB registers

Change-Id: I004629435bca409e7fcd1d0c5d5877626046c34c
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>