Add dcr_host_t.base in dcr_read()/dcr_write()

Now that all users of dcr_read()/dcr_write() add the dcr_host_t.base, we
can save them the trouble and do it in dcr_read()/dcr_write().

As some background to why we just went through all this jiggery-pokery,
benh sayeth:

 Initially the goal of the dcr_read/dcr_write routines was to operate like
 mfdcr/mtdcr which take absolute DCR numbers. The reason is that on 4xx
 hardware, indirect DCR access is a pain (goes through a table of
 instructions) and it's useful to have the compiler resolve an absolute DCR
 inline.

 We decided that wasn't worth the API bastardisation since most places
 where absolute DCR values are used are low level 4xx-only code which may
 as well continue using mfdcr/mtdcr, while the new API is designed for
 device "instances" that can exist on 4xx and Axon type platforms and may
 be located at variable DCR offsets.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Jeff Garzik <jeff@garzik.org>