hikey: migrate to bl2_el3

Since non-TF ROM is used in HiKey platform (Hisilicon Hi6220 SoC),
replace BL1 by BL2_EL3 in normal boot mode.

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

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