bdinfo: Don't print out empty DRAM banks

There is no sense in printing out DRAM banks of size 0 since this means they
are empty. Skip them.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>