usb: dwc2: Fix build warning when CONFIG_PM_SLEEP=n

Building with bcm2835_defconfig, which has CONFIG_PM_SLEEP=n causes the
following build warning:

drivers/usb/dwc2/platform.c:227:12: warning: 'dwc2_suspend' defined but not used [-Wunused-function]
drivers/usb/dwc2/platform.c:237:12: warning: 'dwc2_resume' defined but not used [-Wunused-function]

Annotate these functions with '__maybe_unused' to prevent the warnings.

Reported-by: Olof's autobuilder <build@lixom.net>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>