hikey960: migrate to bl2_el3

Since non-TF ROM is used in HiKey960 platform (Hisilicon Hi3660 SoC),
replace BL1 by BL2_EL3 in normal boot mode.

When flush images in recovery mode, keep to use BL1.

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>