boot: zephyr: enable watchdog feed by default

Since we have support for nRF and STM32 families,
and non-vendor watchdog implementation let's enable
this functionality by default.
Imply NRFX_WDT && NRFX_WDT0 && NRFX_WDT1 only when
SOC_FAMILY_NRF is being used.

Signed-off-by: Bartosz Bilas <bartosz.bilas@hotmail.com>