watchdog: Handle SPL build with watchdog disabled

This patch adds some checks, so that the watchdog can be enabled in main
U-Boot proper but can be disabled in SPL.

This will be used by some AT91SAM based boards, which might enable the
watchdog in the main U-Boot proper and not in SPL. It will be enabled in
SPL by default there, so no need to configure it there. This approach
saves some space in SPL.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Andreas Bießmann <andreas@biessmann.org>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Tested on the taurus board:
Tested-by: Heiko Schocher <hs@denx.de>