armv8: Flush dcache before switching to EL2

For ARMv8, U-boot has been running at EL3 with cache and MMU enabled.
Without proper setup for EL2, cache and MMU are both disabled (out of
reset). Before switching, we need to flush the dcache to make sure the
data is in the main memory.

Signed-off-by: York Sun <yorksun@freescale.com>
Acked-by: David.Feng <fenghua@phytium.com.cn>

arch/arm/lib/bootm.c

@@ -199,6 +199,7 @@ static void do_nonsec_virt_switch(void)
 
 #ifdef CONFIG_ARM64
 	smp_kick_all_cpus();
+	flush_dcache_all();	/* flush cache before swtiching to EL2 */
 	armv8_switch_to_el2();
 #ifdef CONFIG_ARMV8_SWITCH_TO_EL1
 	armv8_switch_to_el1();