usb: dwc3: Enable newer refclk-based SOF/ITP generation

Add support for refclk-based SOF and ITP generation, which is
present on DWC3 cores starting in revision 2.50a. This supersedes
the functionality provided by GCTL.SOFTITPSYNC which handled ITP
generation only. This introduces the new GFLADJ register and
writes its fields based on the reference clock period to the
core, which can now be specified as a device tree parameter.

Change-Id: I2c57586cd8b5275b77ebe64cf0835617f0340b62
Signed-off-by: Jack Pham <jackp@codeaurora.org>