ahci: Add and use AHCI_PREG_SCTL_IPM_NODEVSLP definition from ahci-1.3.1.

ahci: Add and use AHCI_PREG_SCTL_IPM_NODEVSLP definition from ahci-1.3.1.

* The DevSleep interface power saving state was introduced with the
  ahci-1.3.1 specification. However actual DevSleep support needs to be
  recognized via the AHCI_REG_CAP2_SDS bit, since many controllers
  identifying as ahci-1.3 already support DevSleep.

* Since the correct bitmask to use in the AHCI_PREG_SCTL_IPM field
  for disabling all power-management features depends on DevSleep support,
  store that precomputed value in sc->sc_ipm_disable.

* Make some operations on the AHCI_PREG_SCTL_IPM_* bitfield a bit clearer.

* While there add AHCI_PREG_CMD_ICC_DEVSLEEP definition.