fix(nxp/common/setup): fix total dram size checking

total_dram_size should be signed value because it is equal to return
value of init_ddr(), so if it is lower or equal zero, report
error as DDR is not initialized correctly.

Signed-off-by: Jiafei Pan <Jiafei.Pan@nxp.com>
Change-Id: Idbc40da103f60f10cb18c5306e97b764c1a9d372
diff --git a/plat/nxp/common/setup/include/plat_common.h b/plat/nxp/common/setup/include/plat_common.h
index 97a9cb7..484ddcf 100644
--- a/plat/nxp/common/setup/include/plat_common.h
+++ b/plat/nxp/common/setup/include/plat_common.h
@@ -113,7 +113,7 @@
 
 typedef struct {
 	uint64_t num_dram_regions;
-	uint64_t total_dram_size;
+	int64_t total_dram_size;
 	region_info_t region[NUM_DRAM_REGIONS];
 } dram_regions_info_t;