disk: define HAVE_BLOCK_DEVICE if CONFIG_CMD_PART

Various code that is conditional upon HAVE_BLOCK_DEVICE is required by
code conditional upon CONFIG_CMD_PART. So, enable HAVE_BLOCK_DEVICE if
CONFIG_CMD_PART is enabled.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Tom Rini <trini@ti.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>