watchdog: move initr_watchdog() to wdt-uclass.c

This function is a bit large for an inline function, and for U-Boot
proper, it is called via a function pointer anyway (in board_r.c), so
cannot be inlined.

It will shortly set a global variable to be used by the
watchdog_reset() function in wdt-uclass.c, so this also allows making
that variable local to wdt-uclass.c.

The WATCHDOG_TIMEOUT_SECS define is not used elsewhere.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Stefan Roese <sr@denx.de>