arm: at91: wdt: Convert watchdog driver to dm/dt

Convert the Watchdog driver for AT91SAM9x processors to support
the driver model and device tree. Changes "CONFIG_AT91SAM9_WATCHDOG"
to new "CONFIG_WDT_AT91" Kconfig option.

Signed-off-by: Prasanthi Chellakumar <prasanthi.chellakumar@microchip.com>
diff --git a/doc/README.at91 b/doc/README.at91
index 6741213..39dd563 100644
--- a/doc/README.at91
+++ b/doc/README.at91
@@ -171,4 +171,4 @@
 	your code (make sure not to disable it in AT91Bootstrap for instance).
 
 	In the U-Boot configuration, the AT91 watchdog support is enabled using
-	the CONFIG_AT91SAM9_WATCHDOG and CONFIG_HW_WATCHDOG options.
+	the CONFIG_WDT and CONFIG_WDT_AT91 options.