rockchip: rk8xx: allocate priv structure for driver

The rk8xx_priv structure need to allocate for driver, or else
it will cause data abort when CPU access it.

This is a bug fix for below patch set:
https://www.mail-archive.com/u-boot@lists.denx.de/msg247345.html



Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>