arm: kirkwood: iconnect: give meaningful names to pinmux configs

The Kirkwood iConnect Device Tree is currently using totally
meaningless names for the pinmux configuration: pmx_gpio_XY.

This patch fixes that by using some more meaningful names such as
pmx_button_power.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>