irqchip: Add support for Layerscape external interrupt lines

irqchip: Add support for Layerscape external interrupt lines

The LS1021A allows inverting the polarity of six interrupt lines
IRQ[0:5] via the scfg_intpcr register, effectively allowing
IRQ_TYPE_LEVEL_LOW and IRQ_TYPE_EDGE_FALLING for those. We just need to
check the type, set the relevant bit in INTPCR accordingly, and fixup
the type argument before calling the GIC's irq_set_type.

In fact, the power-on-reset value of the INTPCR register on the LS1021A
is so that all six lines have their polarity inverted. Hence any
hardware connected to those lines is unusable without this: If the line
is indeed active low, the generic GIC code will reject an irq spec with
IRQ_TYPE_LEVEL_LOW, while if the line is active high, we must obviously
disable the polarity inversion (writing 0 to the relevant bit) before
unmasking the interrupt.

Some other Layerscape SOCs (LS1043A, LS1046A) have a similar feature,
just with a different number of external interrupt lines (and a
different POR value for the INTPCR register). This driver should be
prepared for supporting those by properly filling out the device tree
node. I have the reference manuals for all three boards, but I've only
tested the driver on an LS1021A.

Unfortunately, the Kconfig symbol ARCH_LAYERSCAPE only exists on
arm64, so do as is done for irq-ls-scfg-msi.c: introduce a new symbol
which is set when either ARCH_LAYERSCAPE or SOC_LS1021A is set.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20191107122115.6244-3-linux@rasmusvillemoes.dk