image: fdt: handle coalesced reserve region

Handle in boot_fdt_reserve_region
any return value > 0 of lmb_reserve() function;
it occurs when coalesced region are found:
adjacent reserved region are merged.

This patch avoid the error trace:
  ERROR: reserving fdt memory region failed..
when reserved region are merged (return value = 1).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/common/image-fdt.c b/common/image-fdt.c
index 94089b2..01186ae 100644
--- a/common/image-fdt.c
+++ b/common/image-fdt.c
@@ -71,10 +71,10 @@
 static void boot_fdt_reserve_region(struct lmb *lmb, uint64_t addr,
 				    uint64_t size)
 {
-	int ret;
+	long ret;
 
 	ret = lmb_reserve(lmb, addr, size);
-	if (!ret) {
+	if (ret >= 0) {
 		debug("   reserving fdt memory region: addr=%llx size=%llx\n",
 		      (unsigned long long)addr, (unsigned long long)size);
 	} else {