ipc: Added platform data structure

Since the same ipc driver can be used by many platforms, using
macros for defining ipc_base and i2c_base addresses is not
a scalable approach. So added a platform data structure to pass
this information.

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Acked-by: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Cc: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>