riscv: Fix linking error when building u-boot-spl with no SMP support

Switch off SMP support when building u-boot-spl would cause linking error as follow:
undefined reference to 'secondary hart relocate' and 'smp_call_function'.
Add macro to wrap up proper code region that needs SMP configuration on.

Signed-off by: Leo Liang <ycliang@andestech.com>
Cc: rick@andestech.com
Reviewed-by: NBin Meng <bin.meng@windriver.com>

arch/riscv/cpu/start.S

@@ -193,6 +193,7 @@ spl_stack_gd_setup:
 	mv	sp, s0
 #endif
 
+#if CONFIG_IS_ENABLED(SMP)
 	/* set new stack and global data pointer on secondary harts */
 spl_secondary_hart_stack_gd_setup:
 	la	a0, secondary_hart_relocate
@@ -207,6 +208,7 @@ spl_secondary_hart_stack_gd_setup:
 	la	a0, secondary_harts_relocation_error
 	jal	printf
 	jal	hang
+#endif
 
 	/* set new global data pointer on main hart */
 1:	mv	gp, s0