target/cortex_a: Extract code to read/write from/to register to/from DCC

target/cortex_a: Extract code to read/write from/to register to/from DCC

In preparation for supporting the ARM MCRR and MRRC commands which will
require using two 32-bit registers to read/write a 64-bit internal
register, extract the common logic to read/write from/to a register
to/from DCC and make that parameterized such that we can do this through
not just r0.

Change-Id: Iadb73f5cde8cf5961b5a18ddd198bf39d791e610
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-on: http://openocd.zylin.com/5227
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>