xlat: remove cast in MAP_REGION to get back building with GCC 4.9

Since commit 769d65da77 ("xlat: Use MAP_REGION macro as compatibility
layer"), building with GCC 4.9 fails.

  CC      plat/arm/board/fvp/fvp_common.c
plat/arm/board/fvp/fvp_common.c:60:2: error: initializer element is not constant
  ARM_MAP_SHARED_RAM,
  ^
plat/arm/board/fvp/fvp_common.c:60:2: error: (near initialization for 'plat_arm_mmap[0]')
make: *** [Makefile:535: build/fvp/release/bl1/fvp_common.o] Error 1

Taking into account that MAP_REGION(_FLAT) is widely used in array
initializers, do not use cast.

Fixes: 769d65da77 ("xlat: Use MAP_REGION macro as compatibility layer")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

include/lib/xlat_tables/xlat_tables_v2.h

@@ -23,12 +23,12 @@
 /* Helper macro to define entries for mmap_region_t. It allows to
  * re-map address mappings from 'pa' to 'va' for each region.
  */
-#define MAP_REGION(_pa, _va, _sz, _attr) ((mmap_region_t){	\
+#define MAP_REGION(_pa, _va, _sz, _attr) {			\
 	.base_pa = (_pa),					\
 	.base_va = (_va),					\
 	.size    = (_sz),					\
 	.attr    = (_attr),					\
-	})
+	}
 
 /*
  * Shifts and masks to access fields of an mmap_attr_t