arm: rockchip: Add common cru.h

Few of the rockchip family SoC atleast rk3288,
rk3399 are sharing some cru register bits so
adding common code between these SoC families
would require to include both cru include files
that indeed resulting function declarations error.

So, create a common cru include as cru.h then
include the rk3399 arch cru include file and move
the common cru register bit definitions into it.

The rest of rockchip cru files will add it in future.

Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>