sandbox, x86: select DM_KEYBOARD instead of default y entry

Once we migrate to DM-based drivers, we cannot go back to legacy
ones, i.e. config options like DM_* are not user-configurable.

Make SANDBOX and X86 select DM_KEYBOARD like other platforms do.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>