soc/intel/common: Add reset_prepare() for common reset

Some Intel SoC may need preparation before reset can be properly
handled. Add callback that chip/soc code can implement.

BUG=chrome-os-partner:55055

Change-Id: I45857838e1a306dbcb9ed262b55e7db88a8944e5
Signed-off-by: Andrey Petrov <andrey.petrov@intel.com>
Reviewed-on: https://review.coreboot.org/15720
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/include/reset.h

@@ -10,5 +10,6 @@ void soft_reset(void);
 void cpu_reset(void);
 /* Some Intel SoCs use a special reset that is specific to SoC */
 void global_reset(void);
-
+/* Some Intel SoCs may need to prepare/wait before reset */
+void reset_prepare(void);
 #endif