devicetree: replace DT_HAS_DRV_INST with DT_INST_FOREACH

Make drivers multi-instance wherever possible using DT_INST_FOREACH.
This allows removing DT_HAS_DRV_INST in favor of making drivers just
do the right thing regardless of how many instances there are.

There are a few exceptions:

- SoC drivers which use CMake input files (like i2c_dw.c) or otherwise
  would require more time to convert than I have at the moment. For the
  sake of expediency, just inline the DT_HAS_DRV_INST expansion for
  now in these cases.

- SoC drivers which are explicitly single-instance (like the nRF SAADC
  driver). Again for the sake of expediency, drop a BUILD_ASSERT in
  those cases to make sure the assumption that all supported SoCs have
  at most one available instance is valid, failing fast otherwise.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

drivers/watchdog/wdt_esp32.c

@@ -270,10 +270,10 @@ static void wdt_esp32_isr(struct device *dev)
 }
 
 
-#if DT_HAS_DRV_INST(0)
+#if DT_HAS_NODE_STATUS_OKAY(DT_NODELABEL(wdt0))
 ESP32_WDT_INIT(0);
 #endif
 
-#if DT_HAS_DRV_INST(1)
+#if DT_HAS_NODE_STATUS_OKAY(DT_NODELABEL(wdt1))
 ESP32_WDT_INIT(1);
 #endif