cpu: Add get_vendor callback

The CPU udevice already has a few callbacks to retreive information
about the currently running CPUs. This patch adds a new get_vendor()
call that returns the vendor of the main CPUs.

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