[POWERPC] Fixes to allow use of Ebony's flash chips through physmap_of

This patch contains a handful of small fixes to allow the Ebony's
flash to be exposed as MTD devices via the physmap_of driver.
Specifically it:
	- Makes a small addition to the device tree and zImage wrapper
to record the correct address for the flash in the device tree based
on the board switches as reported via an FPGA register.
	- Prohibits building the old hard-coded "Ebony" flash map on
arch/powerpc kernels, in favour of using physmap_of's device tree
based approach.
	- Enables MTD and physmap_of in the Ebony defconfig.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>