cpu/ppc4xx/fdt.c: avoid strcpy() to constant string

strcpy() was iused with the target address being a pointer to a
constant string, which potentially is read-only. Use a (writable)
array of characters instead.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>