irqchip/gic-v3-its: Make free_lpi_range a little cheaper

Using list_add + list_sort to insert an element and keeping the list
sorted is a somewhat blunt instrument; one can find the right place to
insert in fewer lines of code than the cmp callback uses. Moreover,
walking the entire list afterwards to merge adjacent ranges is
overkill, since we know that only the just-inserted element may be
merged with its neighbours.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>