usb: dwc3: Add tuning support for Gen2 Tx compliance parameters

This change provides the infrastructure required to tune Gen2
Tx compliance parameters via device tree. The tuning values
provided through dt properties are written to the corresponding
de-emph dwc3 registers during core initialization.

Change-Id: I38015a6f12ca27247bce5ea805d86322122f3466
Signed-off-by: Elson Roy Serrao <eserrao@codeaurora.org>