powerpc/dt_cpu_ftrs: Make use of macro ISA_V3_1

Macro ISA_V3_1 was defined but never used.  Use it instead of literal.

Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200610215114.167544-4-muriloo@linux.ibm.com