cmd_fdt.c: Use %p when printing pointers

When putting pointers into a format string use %p to ensure that they
are printed correctly regardless of bitsize.  This fixes warnings on
sandbox on 64bit systems.

Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Gerald Van Baren <vanbaren@cideas.com>
Signed-off-by: Tom Rini <trini@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>