at91: Minor tweaks to SPL logic for space savings on smartweb

- spl_board_init is empty on smartweb so drop that function
- When CONFIG_AT91SAM9_WATCHDOG is set we do not disable the watchdog in
  SPL and instead let full U-Boot handle it.  Instead of an empty
  function just do not call a function.

Signed-off-by: Tom Rini <trini@konsulko.com>