- Current driver only allows to define a fixed timeout value in the
  board config file.
- Extend 'imx_watchdog_init' parameter list with timeout value as
  received by 'imx_wdt_start' function. Calculate timeout value and
  write appropriate value into WCR register.
- For legacy use, the fixed CONFIG_WATCHDOG_TIMEOUT_MSECS from board
  config is still evaluated.

Signed-off-by: Thomas Schaefer <thomas.schaefer@kontron.com>