usb: dwc3: add lpm erratum support

When parameter DWC_USB3_LPM_ERRATA_ENABLE is enabled in Andvanced
Configuration of coreConsultant, it supports of xHCI BESL Errata Dated
10/19/2011 is enabled in host mode. In device mode it adds the capability
to send NYET response threshold based on the BESL value received in the LPM
token, and the threhold is configurable for each soc platform.

This patch adds an entry that soc platform is able to define the lpm
capacity with their own device tree or bus glue layer.

[ balbi@ti.com : added devicetree documentation, spelled threshold
		completely, made sure threshold is only applied to
		proper core revisions. ]

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