ARM64: poplar: hi3798cv200: u-boot support for Poplar 96Boards

This port adds support for:
1) Serial
2) eMMC
3) USB

It has been tested with ARM TRUSTED FIRMWARE running u-boot as the
BL33 executable [see board's README]

eMMC has been tested for reading and booting the loader and linux
kernels as well as saving the u-boot environment.

USB has been tested with ASIX networking adapter and SanDisk 7.4GB
drive.

PSCI has been tested via the reset call (PSCI executes from DDR)

The firwmare upgrade process has been tested via TFTP and USB FAT
filesystem containing the fastboot.bin image in one of the partitions.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>