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/lowlevel_init.S b/board/raspberrypi/rpi/lowlevel_init.S
index 435eed5..8c39b3e 100644
--- a/board/raspberrypi/rpi/lowlevel_init.S
+++ b/board/raspberrypi/rpi/lowlevel_init.S
@@ -6,15 +6,6 @@
 
 #include <config.h>
 
-.align 8
-.global fw_dtb_pointer
-fw_dtb_pointer:
-#ifdef CONFIG_ARM64
-	.dword 0x0
-#else
-	.word 0x0
-#endif
-
 /*
  * Routine: save_boot_params (called after reset from start.S)
  * Description: save ATAG/FDT address provided by the firmware at boot time
@@ -28,7 +19,8 @@
 	adr	x8, fw_dtb_pointer
 	str	x0, [x8]
 #else
-	str	r2, fw_dtb_pointer
+	ldr	r8, =fw_dtb_pointer
+	str	r2, [r8]
 #endif
 
 	/* Returns */