dm: core: Add Kconfig for simple bus driver

Add Kconfig entries for the simple-bus driver, both for U-Boot
and for SPL. The simple-bus is enabled by default in U-Boot and
disabled by default in SPL to preserve the original behavior.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
Modified to fit on top of Masahiro's $(SPL) setup:
Signed-off-by: Simon Glass <sjg@chromium.org>