device-tree-gpio-naming: RTC battery voltage read enable pin

Add pin used to enable RTC battery voltage reading
(rtc-battery-voltage-read-enable).

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Change-Id: I91e05cb4c947c1dc4a736653ef8c6a0543bbc166
diff --git a/designs/device-tree-gpio-naming.md b/designs/device-tree-gpio-naming.md
index 63b3500..0cec8c4 100644
--- a/designs/device-tree-gpio-naming.md
+++ b/designs/device-tree-gpio-naming.md
@@ -104,6 +104,9 @@
 condition is present. The status will reflect a standby regulator power faulted
 condition when an unexpected drop in standby power is detected.
 
+#### rtc-battery-voltage-read-enable
+Output pin that enables the ADC to read the board RTC battery voltage.
+
 ### Presence
 Pattern: `presence-*`