ARM: hikey: Simplify README instructions.

ARM: hikey: Simplify README instructions.

This patch updates and simplifies the hikey README. The old
instructions were hard to follow, and convoluted.

This patch also updates the link to the mcuimage.bin which was outdated.

Using an outdated mcuimage.bin results in the additional a53 cores
not coming online when the kernel issues PSCI requests to
arm trusted firmware (ATF).

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>