feat(rmm): fix update_ripas()

This patch fixes the following issues:
- smc_rtt_init_ripas() didn't return error if the s2tte was
  assigned_empty
- smc_rtt_set_ripas() failed to properly do these transitions:
  - unassigned_ram -> unassigned_empty
  - assigned_empty -> unassigned_ram
- renames "ripas" variable/structure member of "enum ripas" type
  to "ripas_val".

Signed-off-by: AlexeiFedorov <Alexei.Fedorov@arm.com>
Change-Id: I8ffc5d9c567786c9ae821116441c8dc3178e0242
diff --git a/runtime/core/exit.c b/runtime/core/exit.c
index 098ac96..cd42e20 100644
--- a/runtime/core/exit.c
+++ b/runtime/core/exit.c
@@ -91,7 +91,7 @@
 {
 	unsigned long s2tte, *ll_table;
 	struct rtt_walk wi;
-	enum ripas ripas;
+	enum ripas ripas_val;
 	bool ret;
 
 	assert(GRANULE_ALIGNED(ipa));
@@ -113,8 +113,8 @@
 		ret = false;
 		goto out_unmap_ll_table;
 	}
-	ripas = s2tte_get_ripas(s2tte);
-	ret = (ripas == RIPAS_EMPTY);
+	ripas_val = s2tte_get_ripas(s2tte);
+	ret = (ripas_val == RIPAS_EMPTY);
 
 out_unmap_ll_table:
 	buffer_unmap(ll_table);