usb: xhci-plat: Add DT parameter to program xhci imod_interval

XHCI allows interrupt moderation using imod_interval at 250ns
increments. Add DT parameter to specify this imod_value for
targets mainly with single CPU to reduce CPU interrupt loads.
This allows better balance between CPU usage and performance.

CRs-fixed: 1019219
Change-Id: Id479c162da6492caff4dd83de4054fee63b6abc5
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>