usb: phy: Add support to override tuneX parameters

This change adds debugfs based entries to override tuneX parameters
(where X = 1, 2, 3, 4, 5) related to QUSB PHY. tuneX parameters
helps to fine tune QUSB PHY for better compliance and functionality
based on board design.

How to use:
a. to see tuneX values:
cat /sys/kernel/debug/<qusb_phy_device_name>/tune1

b. to update tuneX values:
echo 0x50 > /sys/kernel/debug/<qusb_phy_device_name>/tune1

Updated values will take effect next time qusb_phy_init() is called,
i.e. next cable connect.

Change-Id: I9bce5842a5e878e22c39b6219fee9dbaeebcc786
Signed-off-by: Mayank Rana <mrana@codeaurora.org>