irqchip/gic-v3-its: Fix the incorrect BUG_ON in its_init_vpe_domain()

The driver probe path hits 'BUG_ON(entries != vpe_proxy.dev->nr_ites)'
on systems where it has VLPI capability, doesn't support direct LPI
feature and boot with a single CPU.

Relax the BUG_ON() condition to fix the issue.

Signed-off-by: Shanker Donthineni <shankerd@codeaurora.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>