spapr: fix LSI interrupt specifiers in the device tree

LoPAPR 1.1 B.6.9.1.2 describes the "#interrupt-cells" property of the
PowerPC External Interrupt Source Controller node as follows:

“#interrupt-cells”

  Standard property name to define the number of cells in an interrupt-
  specifier within an interrupt domain.

  prop-encoded-array: An integer, encoded as with encode-int, that denotes
  the number of cells required to represent an interrupt specifier in its
  child nodes.

  The value of this property for the PowerPC External Interrupt option shall
  be 2. Thus all interrupt specifiers (as used in the standard “interrupts”
  property) shall consist of two cells, each containing an integer encoded
  as with encode-int. The first integer represents the interrupt number the
  second integer is the trigger code: 0 for edge triggered, 1 for level
  triggered.

This patch fixes the interrupt specifiers in the "interrupt-map" property
of the PHB node, that were setting the second cell to 8 (confusion with
IRQ_TYPE_LEVEL_LOW ?) instead of 1.

VIO devices and RTAS event sources use the same format for interrupt
specifiers: while here, we introduce a common helper to handle the
encoding details.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Cédric Le Goater <clg@kaod.org>
--
v3: - reference public LoPAPR instead of internal PAPR+ in changelog
    - change helper name to spapr_dt_xics_irq()

v2: - drop the erroneous changes to the "interrupts" prop in PCI device nodes
    - introduce a common helper to encode interrupt specifiers
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>