hw/arm: Express dependencies of the PXA2xx machines with Kconfig

hw/arm: Express dependencies of the PXA2xx machines with Kconfig

Add Kconfig dependencies for the PXA2xx machines (akita, borzoi,
connex and verdex gumstix, tosa, mainstone, spitz, terrier and z2).
This patch is based on earlier work by Ákos Kovács (i.e.
his "hw/arm/Kconfig: Add ARM Kconfig" patch).

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>