usb: musb: Add context save and restore support

Adding support for MUSB register save and restore during system
suspend and resume.

Changes:
        - Added musb_save/restore_context() functions
        - Added platform specific musb_platform_save/restore_context()
          to handle platform specific jobs.
        - Maintaining BlackFin compatibility by adding read/write
          functions for registers which are not available in BlackFin

Tested system suspend and resume on OMAP3EVM board.

Signed-off-by: Anand Gadiyar <gadiyar@ti.com>
Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>