irqchip/gic-v3-its: Rely on genpool alignment

its_create_device() over-allocated by ITS_ITT_ALIGN - 1 bytes to ensure
that an aligned area was available within the allocation. The new genpool
allocator has its min_alloc_order set to get_order(ITS_ITT_ALIGN) so all
allocations from it should be appropriately aligned.

Remove the over-allocation from its_create_device() and alignment from
its_build_mapd_cmd().

Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Will Deacon <will@kernel.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/all/20241002141630.433502-3-steven.price@arm.com