docs/memory.txt: Improve list of different memory regions

Improve the part of the memory region documentation which describes
the various different kinds of memory region:
 * add the missing types ROM, IOMMU and reservation
 * mention the functions used to initialize each type, as a hint
   for finding the API docs and examples of use

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <1454007297-3971-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
