usb: phy: Add HS PHY tuning parameters to debugfs for SNPS PHY

This allows tuning of PHY parameters using debugfs. This helps in
coming up with appropriate parameter values in electrical compliance
testing.

Change-Id: Iaa65d33ee9e0483bcb753dc98257997ae1dffa2e
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>