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.au


Signed-off-by: Joel Stanley <joel@jms.id.au>