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

Since commit 769d65da ("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: 769d65da ("xlat: Use MAP_REGION macro as compatibility layer")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>