net: add hifemac Ethernet driver for HiSilicon platform

It adds the driver for HIFEMAC Ethernet controller found on HiSilicon
SoCs like Hi3798MV200.  It's based on the mainstream linux driver, but
quite a lot of code gets rewritten and cleaned up to adopt u-boot driver
model.

Signed-off-by: Yang Xiwen <forbidden405@outlook.com>