Symbols in spl.c only function correctly in SPL stage. Build the file
for SPL only to avoid weak symbols in proper U-Boot being unexpectedly
reloaded.
Fixes: 5fe9ced3552 ("riscv: cpu: Add TH1520 CPU support") Signed-off-by: Yao Zi <ziyao@disroot.org> Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>