dwc3: Check USB LPM status before accessing any register

USB debugfs entries can be accessed when USB session is not active or
USB is in low power mode. This results into unclocked register access.
Fix this issue by checking USB LPM status.

debugfs_create_regset32() fops related APIs are used to get USB dwc3
register dump. These fops don't have support to query device status
before accessing registers. Hence by default disabling creation of
regdump debugfs entry.

Change-Id: I67b00532eca159e000c848cb056d9759c47aefe9
Signed-off-by: Mayank Rana <mrana@codeaurora.org>