power: Fix naming of Kconfig options controlling low power states

The SYS_POWER_LOW_POWER_STATE_SUPPORTED and SYS_POWER_LOW_POWER_STATE
suggests one low power state but these options control multiple
low power state. This commit uses plural in the names to indicate
that.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>