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/arch/riscv/lib/Makefile b/arch/riscv/lib/Makefile
index adadbf4..bd7b2c4 100644
--- a/arch/riscv/lib/Makefile
+++ b/arch/riscv/lib/Makefile
@@ -22,7 +22,7 @@
 obj-y	+= interrupts.o
 obj-y	+= reset.o
 obj-y   += setjmp.o
-obj-$(CONFIG_SMP) += smp.o
+obj-$(CONFIG_$(SPL_)SMP) += smp.o
 obj-$(CONFIG_SPL_BUILD)	+= spl.o
 
 # For building EFI apps