(Re-)added the Deskpro 386, but only in the development/incomplete section of the code definitions.

Selected XT and AT clones can use either their built-in FDC controller or an external one (the IBM AT and Compaq AT machines don't support booting from a 1.44M floppy so this makes the external floppy useful).
Added the FDC to the Adaptec AHA-154xCF ("2" variant) and defaulted to None to keep compatibility with existing FDC's.

src/machine/m_at.c

@@ -49,6 +49,7 @@
 #include <86box/device.h>
 #include <86box/fdd.h>
 #include <86box/fdc.h>
+#include <86box/fdc_ext.h>
 #include <86box/nvr.h>
 #include <86box/gameport.h>
 #include <86box/keyboard.h>
@@ -102,7 +103,8 @@ machine_at_ibm_common_init(const machine_t *model)
 
     mem_remap_top(384);
 
-    device_add(&fdc_at_device);
+    if (fdc_type == FDC_INTERNAL)
+	device_add(&fdc_at_device);
 }