lib/imd_cbmem.c: Add a helper function to indicate that cbmem is ready

This can be used in romstage in particular to know if dram is ready.

Change-Id: I0231ab9c0b78a69faa762e0a97378bf0b50eebaf
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38736
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/include/cbmem.h

@@ -155,4 +155,18 @@ static inline int cbmem_possibly_online(void)
 	return 1;
 }
 
+/* Returns 1 after running cbmem init hooks, 0 otherwise. */
+static inline int cbmem_online(void)
+{
+	extern int cbmem_initialized;
+
+	if (!cbmem_possibly_online())
+		return 0;
+
+	if (ENV_ROMSTAGE)
+		return cbmem_initialized;
+
+	return 1;
+}
+
 #endif /* _CBMEM_H_ */