hikey: fastboot: update the download region
hikey: fastboot: update the download region

The download image address is changed to 0x2000_0000. And the maximium
download image size is changed to 256MB. Since we need to reserve the
region as filling 0.

Signed-off-by: Haojian Zhuang <979139d45d262f0480136b0da512e0afa0fe79d3@linaro.org>

96boards/arm-trusted-firmware,vchong/arm-trusted-firmware,vchong/arm-trusted-firmware,96boards/arm-trusted-firmware