bus: imx-weim: populate devices on a simple bus

There could be some memory map devices located in
a certain chip select region of the i.MX WEIM.
The devices could be attached to a simple bus(for
example, a AXI bus) whose root node is one child
device tree node of the i.MX WEIM device tree node.
There should be a bridge(very likely, software
transparent) bewteen the i.MX WEIM and the simple bus.
This patch makes the i.MX WEIM driver possible to
populate devices on a simple bus.  In this way, people
may try various IPs(in a FPGA, maybe) outside of i.MX
chips with the i.MX WEIM embedded.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>