riscv: Introduce SPL_SMP Kconfig option for U-Boot SPL

With SBI v0.2 HSM extension, only a single hart need to boot and
enter operating system. The booting hart can bring up secondary
harts one by one afterwards.

For U-Boot running in SPL, SMP can be turned on, while in U-Boot
proper, SMP can be optionally turned off if using SBI v0.2 HSM.

Introduce a new SPL_SMP Kconfig option to support this.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
diff --git a/common/spl/spl_opensbi.c b/common/spl/spl_opensbi.c
index a136073..3519c34 100644
--- a/common/spl/spl_opensbi.c
+++ b/common/spl/spl_opensbi.c
@@ -76,7 +76,7 @@
 	opensbi_entry = (void (*)(ulong, ulong, ulong))spl_image->entry_point;
 	invalidate_icache_all();
 
-#ifdef CONFIG_SMP
+#ifdef CONFIG_SPL_SMP
 	/*
 	 * Start OpenSBI on all secondary harts and wait for acknowledgment.
 	 *