ARM: vGIC: advertise LPI support

ARM: vGIC: advertise LPI support

To let a guest know about the availability of virtual LPIs, set the
respective bits in the virtual GIC registers and let a guest control
the LPI enable bit.
Only report the LPI capability if there is at least one ITS emulated
for that guest (which depends on the host having an ITS at the moment).
For Dom0 we report the same number of interrupts identifiers as the
host, whereas DomUs get a number fixed at 10 bits for the moments, which
covers all SPIs. Also we fix a slight inaccuracy here, since the
number of interrupt identifier specified in GICD_TYPER depends on the
stream interface and is independent from the number of actually wired
SPIs.
This also removes a "TBD" comment, as we now populate the processor
number in the GICR_TYPER register, which will be used by the ITS
emulation later on.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>