powerpc: Remove DCR_MMIO and the DCR generic layer

The Cell blade support was the last user of DCR_MMIO, so it can now
be removed.

That only leaves DCR_NATIVE, meaning the DCR generic layer which allows
using either DCR_NATIVE or DCR_MMIO is also unnecessary, remove it too.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20241218105523.416573-7-mpe@ellerman.id.au