common/board_f.c: Enable IMX watchdog in init_func_watchdog_init()

Without this patch, the IMX watchdog will not be initialized. And therefor
not active. This patch fixes this by calling hw_watchdog_init() also when
CONFIG_IMX_WATCHDOG is defined.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>