uboot-imx

hisilicon: hikey: Update instructions based on latest source

Update the HiKey board instructions based on the latest source
available. These instructions are derived from the ATF platform doc.
While updating the instructions, some comments on ATF issue has been
removed since it is fixed in latest ATF source.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>