linux: Release 4.4.15-openpower1

New patches:
 - powerpc/boot: Add OPAL console to epapr wrappers
 - net/mlx5: Add pci shutdown callback, for kexec support
 - tty/hvc: Use IRQF_SHARED for OPAL hvc consoles
 - tty/hvc: Use opal irqchip interface if available

Reworked due to upstream fixes:
 - xhci: do not halt the secondary HCD

Superseded by upstream fixes:
 - Revert "usb: xhci: stop everything on the first call to xhci_stop"
 - xhci: do not halt the secondary HCD

Signed-off-by: Joel Stanley <joel@jms.id.au>
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