dm: core: Enable SPL_SIMPLE_BUS by default

This option is needed for all SoCs which have nodes on bus. Without
enabling this drivers are not found and probed.
Issue was found on Zynq MMC probe.
Enable this option by default.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>