ARM: at91: pm: allow selecting standby and suspend modes

While we can only select between "standby" and "mem" states for power
management, the atmel platforms can actually support more modes.

For both standby and mem, allow selecting which mode will be used using the
atmel.pm_modes kernel parameter.
By default, keep the current modes.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Wenyou Yang <wenyou.yang@atmel.com>