bdinfo: ppc: Move PPC-specific info into its own file

We don't really want to have PPC-specific code in a generic file. Create
a new arch-specific function to hold it, and move it into that.

Make the function weak so that any arch can implement it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>