KVM: arm64: Convert pkvm_mappings to interval tree

In preparation for supporting stage-2 huge mappings for np-guest, let's
convert pgt.pkvm_mappings to an interval tree.

No functional change intended.

Suggested-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Link: https://lore.kernel.org/r/20250521124834.1070650-8-vdonnefort@google.com


Signed-off-by: Marc Zyngier <maz@kernel.org>