x86/ioapic: Cleanup IO/APIC route entry structs

Having two seperate structs for the I/O-APIC RTE entries (non-remapped and
DMAR remapped) requires type casts and makes it hard to map.

Combine them in IO_APIC_routing_entry by defining a union of two 64bit
bitfields. Use naming which reflects which bits are shared and which bits
are actually different for the operating modes.

[dwmw2: Fix it up and finish the job, pulling the 32-bit w1,w2 words for
        register access into the same union and eliminating a few more
        places where bits were accessed through masks and shifts.]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201024213535.443185-21-dwmw2@infradead.org