soc/lowrisc: Place CBMEM at top of autodetected RAM

Change-Id: I9f169db330d1792128db7a6308ed3afbe5803c03
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/17708
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/soc/lowrisc/lowrisc/cbmem.c

@@ -11,13 +11,15 @@
  * GNU General Public License for more details.
  */
 
-#include <console/console.h>
 #include <cbmem.h>
+#include <commonlib/configstring.h>
 
 void *cbmem_top(void)
 {
-	// TODO: find out how the lowrisc SOC decides to provide
-	// this information, when they know.
-	printk(BIOS_SPEW, "Returning hard-coded 128M; fix me\n");
-	return (void *)((uintptr_t)(2ULL*GiB+128*MiB));
+	uintptr_t base;
+	size_t size;
+
+	query_mem(configstring(), &base, &size);
+
+	return (void *)(base + size);
 }