gpu: vivante: ETNAVIV: Add new driver.

This new driver is a port of the <https://github.com/laanwj/etna_viv>
Project Etnaviv. It is an attempt to create an opensource driver and
userspace for Vivante GCxxx series of embedded GPUs. Support for the
Ingenic JZ4770 on the GCW0 has been added.

Signed-off-by: Steven J. Hill <sjhill@realitydiluted.com>