watchdog: introduce separate SPL symbol for WDT_GPIO

watchdog: introduce separate SPL symbol for WDT_GPIO

Currently, enabling WDT_GPIO on a board which uses SPL, but does not
have SPL_WDT, SPL_DM_GPIO or SPL_OF_CONTROL enabled, breaks the build.

Make it possible to use the WDT_GPIO driver on such boards by
introducing a separate symbol controlling whether the driver is built
for SPL. Make it default to WDT_GPIO such that boards that already
have it enabled and all the SPL prerequisites satisfied will continue
to have it in SPL.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Stefan Roese <sr@denx.de>