dm: Add dm_remove_devices_active() for ordered device removal

dm: Add dm_remove_devices_active() for ordered device removal

This replaces dm_remove_devices_flags() calls in all boot
implementations to ensure non vital devices are consistently removed
first. All boot implementation except arch/arm/lib/bootm.c currently
just call dm_remove_devices_flags(DM_REMOVE_ACTIVE_ALL). This can result
in crashes when dependencies between devices exists. The driver model's
design document describes DM_FLAG_VITAL as "indicates that the device is
'vital' to the operation of other devices". Device removal at boot
should follow this.

Instead of adding dm_remove_devices_flags() with (DM_REMOVE_ACTIVE_ALL |
DM_REMOVE_NON_VITAL) everywhere add dm_remove_devices_active() which
does this.

Fixes a NULL pointer deref in the apple dart IOMMU driver during EFI
boot. The xhci-pci (driver which depends on the IOMMU to work) removes
its mapping on removal. This explodes when the IOMMU device was removed
first.

dm_remove_devices_flags() is kept since it is used for testing of
device_remove() calls in dm.

Signed-off-by: Janne Grunau <j@jannau.net>