bdinfo: xtensa: Create a generic do_bdinfo for xtensa

bdinfo: xtensa: Create a generic do_bdinfo for xtensa

This arch uses only the generic function. It would be nice if all the
archs did the same. As a first step, create a new generic function for the
'bd' command and make xtensa use it.

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