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>