usb: dwc3: add Tx de-emphasis quirk

This patch adds Tx de-emphasis quirk, and the Tx de-emphasis value is
configurable according to PIPE3 specification.

Value		Description
0		-6dB de-emphasis
1		-3.5dB de-emphasis
2		No de-emphasis
3		Reserved

It can be configured on DT or platform data.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>