Are they (Rockchip) now to become a customer of Intel's SOCs...?
Rockchip will design a "quad-core+3G". While Intel will be busy with "dual-core+3g" and "quad-core + LTE". So Intel is using Rockchip's design group to develop this new part. Of course, the other main benefit is the customer relationships that Rockchip already has.
From AnandTech:
With the new quad-core part, Intel's SoFIA family will be made up of three SoCs: a dual-core part with integrated 3G launching later this year, a quad-core part with integrated LTE in the first half of next year, and the quad-core + 3G part from the Rockchip partnership also in the first half of next year.