ARM: dts: ixp4xx: Create a proper expansion bus

ARM: dts: ixp4xx: Create a proper expansion bus

The IXP4xx expansion bus is 24 bits (256 MB) that is memory
mapped between 0x50000000-0x5fffffff usin a set of chip
selects. The size of the windows is 16 or 32MB defined by
the boot loader system configuration at runtime.

Create a rudimentary simple-bus and move the flash memories
to the expansion bus, inside the SoC.

Cc: Zoltan HERPAI <wigyori@uid0.hu>
Cc: Raylynn Knight <rayknight@me.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>