Makefile: Add missing .dep files for fw_*.elf.ld

Since we don't currently create these, changes to fw_base.ldS do not
cause the preprocessed fw_*.elf.ld files to be rebuilt, and thus
incremental builds can end up failing with missing symbols if crossing
the recent commits that introduced _fw_rw_offset and then replaced it
with _fw_rw_start.

Reported-by: Ben Dooks <ben.dooks@sifive.com>
Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
Reviewed-by: Anup Patel <anup@brainfault.org>