lib/cbmem: provide optional cbmem top initialization hook

Provide a hook to allow an optional one-time cbmem_top() initialization.
The new function, cbmem_top_init(), is called on the first expected
initialization of cbmem based on the Kconfig options LATE_CBMEM_INIT
and EARLY_CBMEM_INIT.

Change-Id: I89edd2d11f226217c8e2aaca829b4f375a2cff28
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Signed-off-by: John Zhao <john.zhao@intel.com>
Reviewed-on: https://review.coreboot.org/20847
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/include/cbmem.h

@@ -64,6 +64,11 @@ int cbmem_initialize_id_size(u32 id, u64 size);
 void cbmem_initialize_empty(void);
 void cbmem_initialize_empty_id_size(u32 id, u64 size);
 
+/* Optional hook for platforms to initialize cbmem_top() value. When employed
+ * it's called a single time during boot at cbmem initialization/recovery
+ * time. */
+void cbmem_top_init(void);
+
 /* Return the top address for dynamic cbmem. The address returned needs to
  * be consistent across romstage and ramstage, and it is required to be
  * below 4GiB.