sim: ppc: fix Wpointer-sign warning

sim: ppc: fix Wpointer-sign warning

When compiling with --enable-werror and CFLAGS="-O0 -g -Wall", we run into:
...
src/sim/ppc/hw_memory.c: In function 'hw_memory_init_address':
src/sim/ppc/hw_memory.c:194:75: error: pointer targets in passing \
  argument 4 of 'device_find_integer_array_property' differ in signedness \
  [-Werror=pointer-sign]
     int nr_cells
       = device_find_integer_array_property(me, "available", 0, &dummy);
                                                                ^
...

Fix this by changing the type of dummy.