usb: dwc2: embed storage for reg backup in struct dwc2_hsotg

Register backup function can be called from atomic context. Instead
of using atomic memory pool, embed backup storage space in
struct dwc2_hsotg.

Also add a valid flag in each struct as NULL pointer can't be used as
the content validity check any more.

Acked-by: John Youn <johnyoun@synopsys.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>