cmd/internal/obj/ppc64: remove unused REG_DCR types

The assembler does not support parsing DCRx registers,
nor does the compiler generate opcodes with these.

Likewise, these registers are only available on ISA
2.07 embedded processors which are not supported in
golang.

Change-Id: Iea258e5958a2022bda0eee8348de1b06437148df
Reviewed-on: https://go-review.googlesource.com/c/go/+/352790


Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>