usb: dwc2: host: use kmem cache to allocate descriptors

Kmem caches help to get correct boundary for descriptor buffers
which need to be 512 bytes aligned for dwc2 controller.
Two kmem caches are needed for generic descriptors and for
hs isochronous descriptors which doesn't have same size.

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>