watchdog: Add support for Xilinx Microblaze watchdog

Watchdog can be used on Microblaze, PPC and Zynq hw designs.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@ti.com>
diff --git a/doc/README.watchdog b/doc/README.watchdog
index ee65008..33f31c2 100644
--- a/doc/README.watchdog
+++ b/doc/README.watchdog
@@ -27,3 +27,6 @@
 	Available for i.mx31/35/5x/6x to service the watchdog. This is not
 	automatically set because some boards (vision2) still need to define
 	their own hw_watchdog_reset routine.
+
+CONFIG_XILINX_TB_WATCHDOG
+	Available for Xilinx Axi platforms to service timebase watchdog timer.