[POWERPC] Re-organize Kconfig code for 4xx in arch/powerpc

Now that we always take a device tree in arch/powerpc, there's no good
reason not to allow a single kernel to support multiple embedded 4xx
boards - the correct platform code can be selected based on the device
tree information.

Therefore, this patch re-arranges the 4xx Kconfig code to allow this.
In addition we:
	- use "select" instead of depends to configure the correct
	  config options for specific 4xx CPUs and workarounds, which
	  makes the information about specific boards and CPUs less
	  scattered.
	- Some old, unused (in arch/powerpc) config options are
	  removed: WANT_EARLY_SERIAL, IBM_OCP, etc.

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>