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/include/asm/global_data.h b/arch/riscv/include/asm/global_data.h
index b74bd7e..72fb4b4 100644
--- a/arch/riscv/include/asm/global_data.h
+++ b/arch/riscv/include/asm/global_data.h
@@ -24,7 +24,7 @@
 #ifdef CONFIG_ANDES_PLMT
 	void __iomem *plmt;	/* plmt base address */
 #endif
-#ifdef CONFIG_SMP
+#if CONFIG_IS_ENABLED(SMP)
 	struct ipi_data ipi[CONFIG_NR_CPUS];
 #endif
 #ifndef CONFIG_XIP