powerpc/boot: Allocate more memory for dtb

David Gibson suggested that since we are now unconditionally copying
the dtb into a malloc()ed buffer, it would be sensible to add a little
padding to the buffer at that point, so that further device tree
manipulations won't need to reallocate it.

This implements that suggestion.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>