ARM: dts: aspeed: rainier: Add gpio line names

Name the GPIOs to help userspace work with them. The names describe the
functionality the lines provide, not the net or ball name. This makes it
easier to share userspace code across different systems and makes the
use of the lines more obvious.

Signed-off-by: Andrew Geissler <geisonator@yahoo.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>