omap_gpmc: Clean up omap_gpmc_attach MemoryRegion conversion

omap_gpmc: Clean up omap_gpmc_attach MemoryRegion conversion

Now that all callers of omap_gpmc_attach pass in a MemoryRegion*,
we can remove the base_update and unmap function pointer arguments,
and the opaque pointer that was passed into these callbacks.

We can also remove the base and size fields from omap_gpmc_cs_file_s
as these are no longer necessary (you don't need the base/size
to unmap a MemoryRegion the way you did to undo a mapping made
with cpu_register_physical_memory()).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>