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>