ARM: dts: aspeed: rainier: gpio-keys for PSU presence

Add in a gpio-keys section to the Rainier device tree source, add in the
power supply presence GPIOs.

Signed-off-by: Brandon Wyman <bjwyman@gmail.com>
Reviewed-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>