[POWERPC] bootwrapper: Use `unsigned long' for malloc sizes

Use `unsigned long' for malloc sizes, to match common practice and types used
by most callers and callees.
Also use `unsigned long' for integers representing pointers in simple_alloc.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@eu.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>