ARM: at91: pm: remove unnecessary at91sam9x60_idle

cpu_do_idle() is already the default action for arm_pm_idle, there is no
need to open code it.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20200804115622.63232-1-alexandre.belloni@bootlin.com