usb: phy: snps: Add debugfs support to tune tx compliance

Debugfs entries will be used to test various tx parameters such
as pre-emphasis and vref to tune the tx compliance and eye diagram.

Change-Id: I5f09c1525c862acfd004a6a0f3414947dcc1c602
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>