powerpc/dt_cpu_ftrs: Add feature for 2nd DAWR

Add new device-tree feature for 2nd DAWR. If this feature is present,
2nd DAWR is supported, otherwise not.

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200723090813.303838-6-ravi.bangoria@linux.ibm.com