cyclic: Add a symbol for SPL

cyclic: Add a symbol for SPL

The cyclic subsystem is currently enabled either in all build phases
or none. For tools this should not be enabled, but since lib/shc256.c
and other files include watchdog.h in the host build, we must make
sure that it is not enabled there.

Add an SPL symbol so that there is more control of this.

Add an include into cyclic.h so that tools can include this file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Devarsh Thakkar <devarsht@ti.com>
Reviewed-by: Stefan Roese <sr@denx.de>