arch/riscv: Move CBMEM into RAM

CBMEM should be placed at the top of RAM, which can be found by parsing
the configuration string. Configuration string parsing isn't yet
implemented, so I'll hard-code the CBMEM location for now.

Change-Id: If4092d094a856f6783887c062d6682dd13a73b8f
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/15284
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/soc/ucb/riscv/cbmem.c

@@ -16,5 +16,5 @@
 void *cbmem_top(void)
 {
 	// TODO: find out how RISCV stores this.
-        return (void *)0x1fff000;
+        return (void *)((uintptr_t)3*GiB);
 }