arm64: Increase NIRQ to 16k

Ampere Altra in a dual socket configuration has 12 ITSes for the
12 PCIe root complexes. The NIRQ interrupts are statically split
between each child of the gic bus, so here we increase that
value. 16k is enough for

(#cpus * #its * max_pcie_bifurcation) LPIs + (#SPIs and #PPIs)

Reviewed by: jhb
Approved by: scottl (implicit)
MFC after: 1 week
Sponsored by: Ampere Computing
Differential Revision: https://reviews.freebsd.org/D26766