rpi: push fw_dtb_pointer in the .data section

The fw_dtb_pointer was defined in the assembly code, which makes him
live in section .text_rest
Put that's not necessary, we can push the variable in the .data section.

This will prevent relocation errors like:
board/raspberrypi/rpi/rpi.c:317:(.text.board_get_usable_ram_top+0x8):
relocation truncated to fit: R_AARCH64_LDST64_ABS_LO12_NC against symbol
`fw_dtb_pointer' defined in .text section in board/raspberrypi/rpi/built-in.o

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
diff --git a/board/raspberrypi/rpi/rpi.c b/board/raspberrypi/rpi/rpi.c
index 9e0abdd..e84a1db 100644
--- a/board/raspberrypi/rpi/rpi.c
+++ b/board/raspberrypi/rpi/rpi.c
@@ -27,8 +27,11 @@
 
 DECLARE_GLOBAL_DATA_PTR;
 
-/* From lowlevel_init.S */
-extern unsigned long fw_dtb_pointer;
+/* Assigned in lowlevel_init.S
+ * Push the variable into the .data section so that it
+ * does not get cleared later.
+ */
+unsigned long __section(".data") fw_dtb_pointer;
 
 /* TODO(sjg@chromium.org): Move these to the msg.c file */
 struct msg_get_arm_mem {