usb: musb: drop musb_platform_suspend/resume

all glue layers are now fully moved to the
new setup. We are now using dev_pm_ops to
implement suspend/resume functionality and
thus, musb_platform_suspend/resume has become
deprecated and useless.

This patch drops those function pointers and
its uses.

Signed-off-by: Felipe Balbi <balbi@ti.com>