armv8: fix #if around spin-table code in start.S

Using CONFIG_IS_ENABLED() doesn't work in SPL. This patch replaces the only
occurrence of CONFIG_IS_ENABLED() in start.S to a regular #if defined().
It also adds "&& !defined(CONFIG_SPL_BUILD)" to that #if statement because
the spin-table code can't currently work in SPL, and the spin-table file
isn't even compiled in SPL.

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>

arch/arm/cpu/armv8/start.S

@@ -98,7 +98,7 @@ save_boot_params_ret:
 	/* Processor specific initialization */
 	bl	lowlevel_init
 
-#if CONFIG_IS_ENABLED(ARMV8_SPIN_TABLE)
+#if defined(CONFIG_ARMV8_SPIN_TABLE) && !defined(CONFIG_SPL_BUILD)
 	branch_if_master x0, x1, master_cpu
 	b	spin_table_secondary_jump
 	/* never return */