cmd: fdt: Handle 64bit pointers in fdt get addr

The command assumed 32bit pointers so far, with 64bit pointer the
command would overwrite a piece of stack. Fix it by extending the
array size to cater for 64bit pointer, and use snprintf() to avoid
writing past the end of the array ever again.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>