ARM: at91: pm: Use ULP0 naming instead of slow clock

Switch to use ULP0 naming instead of slow clock naming for power modes, to
be as closed as possible to datasheet. This commit does the necessary
renaming and macro addition to be as close as possible to the namings
from [1].

[1] https://lore.kernel.org/lkml/1470650705-31418-3-git-send-email-wenyou.yang@atmel.com

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>