riscv: cpu: th1520: Build spl.c for SPL only

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>

arch/riscv/cpu/th1520/Makefile

@@ -5,4 +5,4 @@
 obj-y += cache.o
 obj-y += cpu.o
 obj-y += dram.o
-obj-y += spl.o
+obj-$(CONFIG_SPL_BUILD) += spl.o