gic_v3: Use 64-bit writes to GICD_IROUTERn

While 32-bit accesses to GICD_IROUTERn are allowed,
this is a 64-bit register.

Pull Request: https://github.com/freebsd/freebsd-src/pull/451

(cherry picked from commit 46ca0b903cad8955574a2c61bed41c2b92aa2273)
(cherry picked from commit 40f283b038015b5b750267a26402098418fd1fb3)

Approved by: re (gjb)