irqchip/gic-v3-its: Share ITS tables with a non-trusted hypervisor

Within a realm guest the ITS is emulated by the host. This means the
allocations must have been made available to the host by a call to
set_memory_decrypted(). Introduce an allocation function which performs
this extra call.

For the ITT use a custom genpool-based allocator that calls
set_memory_decrypted() for each page allocated, but then suballocates the
size needed for each ITT. Note that there is no mechanism implemented to
return pages from the genpool, but it is unlikely that the peak number of
devices will be much larger than the normal level - so this isn't expected
to be an issue.

Co-developed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
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/20241002141630.433502-2-steven.price@arm.com