ARM: dts: fp5280g2: Cleanup gpio-keys-polled properties

dtbs_check gave the following warning:

    Warning (avoid_unnecessary_addr_size): /gpio-keys-polled: unnecessary #address-cells/#size-cells without "ranges" or child "reg" property

Cc: John Wang <wangzqbj@inspur.com>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Adriana Kobylak <anoo@us.ibm.com>
Tested-by: Adriana Kobylak <anoo@us.ibm.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>