watchdog: Allow to use CONFIG_WDT without starting watchdog

In some cases it is useful to compile support for U-Boot command 'wdt'
without starting HW watchdog in early U-Boot phase. For example when the
user want to start the watchdog only on demand by some boot script.

This change adds a new compile option WATCHDOG_AUTOSTART to control whether
U-Boot should automatically start the watchdog during init phase or not.

This option is enabled by default as it was the default behavior prior
introducing this new change. When compiling U-Boot users can decide to turn
this option off.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>