ARM: at91: pm: add support for sama5d2 secure suspend

When running with OP-TEE, the suspend control is handled securely.
Suspend can be entered using PSCI support. Since the sama5d2 supports
multiple suspend modes, add a new CONFIG_ATMEL_SECURE_PM which will
send a SMC call to select the suspend mode at init time.

"atmel.pm_modes" boot argument is still supported for compatibility
purposes but the standby value is actually ignored since PSCI suspend
is used and it only support one mode (suspend).

Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>