board: sifive: compile stuff only related to SPL in SPL build

As (3581811d "riscv: sifive/fu540: Move SPL related functions to spl.c"),
we put the SPL stuff in spl.c, we don't need to compile unleashed.c and
unmatched.c in SPL build.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>