powerpc/pasemi: Add Nemo motherboard config option.

Add config option for the Nemo motherboard used in the Amigaone X1000.
This is a custom PASemi board with an AMD SB600 southbridge, and needs
some patches to it device tree. This option will be used to build these
into the kernel

Signed-off-by: Darren Stevens <darren@stevens-zone.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>