watchdog: Let XILINX_WATCHDOG and TEGRA_WATCHDOG depend on HAS_IOMEM

They need HAS_IOMEM, so let them depend on it, the related error (with
allmodconfig under um):

    MODPOST 1205 modules
  ERROR: "devm_ioremap_resource" [drivers/watchdog/tegra_wdt.ko] undefined!
  ERROR: "devm_ioremap_resource" [drivers/watchdog/of_xilinx_wdt.ko] undefined!

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>