POWERPC: Get rid of remapping the whole immr

The stuff below cleans up the code attempting to remap the whole cpm2_immr
early, as well as places happily assuming that fact. This is more like the 2.4
legacy stuff, and is at least confusing and unclear now.

To keep the world comfortable, a new mechanism is introduced: before accessing
specific immr register/register set, one needs to map it, using cpm2_map(<reg>),
for instance, access to CPM command register will look like
	volatile cpm_cpm2_t *cp = cpm2_map(im_cpm);
keeping the code clear, yet without "already defined somewhere" cpm2_immr.

So far, unmapping code is not implemented, but it's not a big deal to add it,
if the whole idea makes sense.

Signed-off-by: Vitaly Bordug <vbordug@ru.mvista.com>