arm: simplify MMIO dispatching

Currently we separate any incoming MMIO request into one of the ARM
memory map regions and take care to spare the GIC.
It turns out that this is unnecessary, as we only have one special
region (the IO port area in the first 64 KByte). The MMIO rbtree
takes care about unhandled MMIO ranges, so we can simply drop all the
special range checking (except that for the IO range) in
kvm_cpu__emulate_mmio().
As the GIC is handled in the kernel, a GIC MMIO access should never
reach userland (and we don't know what to do with it anyway).
This lets us delete some more code and simplifies future extensions
(like expanding the GIC regions).
To be in line with the other architectures, move the now simpler
code into a header file.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>