usb: musb: dsps: add support for suspend and resume

The dsps platform needs to save save some registers at suspend time and
restore them after resume. This patch adds a struct for these registers,
and also lets the musb core know that the core registers need to be
saved as well.

We also have to explicitly de-assert the port reset upon resume on this
platform, but musb_port_reset() should not be called from glue layers.

Hence, introduce a flag in struct musb_hdrc_config for this.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>