mpic: Unify numbering scheme

mpic: Unify numbering scheme

MPIC interrupt numbers in Linux (device tree) and in QEMU are different,
because QEMU takes the sparseness of the IRQ number space into account.

Remove that cleverness and instead assume a flat number space. This makes
the code easier to understand, because we are actually aligned with Linux
on the view of our worlds.

Signed-off-by: Alexander Graf <agraf@suse.de>