of: property: Fix fw_devlink handling of interrupt-map

Commit d976c6f4 ("of: property: Add fw_devlink support for
interrupt-map property") tried to do what it says on the tin,
but failed on a couple of points:

- it confuses bytes and cells. Not a huge deal, except when it
  comes to pointer arithmetic

- it doesn't really handle anything but interrupt-maps that have
  their parent #address-cells set to 0

The combinations of the two leads to some serious fun on my M1
box, with plenty of WARN-ON() firing all over the shop, and
amusing values being generated for interrupt specifiers.

Having 2 versions of parsing code for "interrupt-map" was a bad
idea. Now that the common parsing parts have been refactored
into of_irq_parse_imap_parent(), rework the code here to use it
instead and fix the pointer arithmetic.

Note that the dependency will be a bit different than the original code
when the interrupt-map points to another interrupt-map. In this case,
the original code would resolve to the final interrupt controller. Now
the dependency is the parent interrupt-map (which itself should have a
dependency to the parent). It is possible that a node with an
interrupt-map has no driver.

Fixes: d976c6f4

 ("of: property: Add fw_devlink support for interrupt-map property")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Co-developed-by: Rob Herring (Arm) <robh@kernel.org>
Cc: Saravana Kannan <saravanak@google.com>
Tested-by: Marc Zyngier <maz@kernel.org>
Tested-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20240529-dt-interrupt-map-fix-v2-2-ef86dc5bcd2a@kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>