BL31: Fix relocation error for PIE

This patch fixes BL31 linker error
 "relocation R_AARCH64_ABS32 against `a local symbol'
 can not be used when making a shared object"
when Position Independent Executable (PIE) support
is enabled with ENABLE_PIE=1 build option.

Change-Id: I2692269676db3f3b27eed499fc029fffb67969be
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>

plat/common/aarch64/platform_mp_stack.S

@@ -59,7 +59,8 @@ func plat_get_my_stack
 	bic	x0, x0, #(CACHE_WRITEBACK_GRANULE - 1)
 	ret	x10
 #endif
-	.word	platform_normal_stacks
+	/* Prevent linker from removal of stack section */
+	.quad	platform_normal_stacks
 
 #else /* !(IMAGE_BL31 && RECLAIM_INIT_CODE) */
 	mov	x10, x30