ARM: dts: aspeed: rainer: Add RTC battery gpio name

This is the documented name used for OpenBMC systems:

 https://github.com/openbmc/docs/blob/master/designs/device-tree-gpio-naming.md#rtc-battery-voltage-read-enable

Link: https://lore.kernel.org/r/20220222041559.68651-2-joel@jms.id.auSigned-off-by: NJoel Stanley <joel@jms.id.au>

arch/arm/boot/dts/aspeed-bmc-ibm-rainier.dts

@@ -250,7 +250,7 @@
 	/*C0-C7*/	"","","","","","","","",
 	/*D0-D7*/	"","","","","","","","",
 	/*E0-E7*/	"","","","","","","","",
-	/*F0-F7*/	"","","","reset-cause-pinhole","","","factory-reset-toggle","",
+	/*F0-F7*/	"","","rtc-battery-voltage-read-enable","reset-cause-pinhole","","","factory-reset-toggle","",
 	/*G0-G7*/	"","","","","","","","",
 	/*H0-H7*/	"","bmc-ingraham0","rear-enc-id0","rear-enc-fault0","","","","",
 	/*I0-I7*/	"","","","","","","bmc-secure-boot","",