ARM: dts: ls1021a: Specify interrupt-affinity for pmu node

This avoids the warning

  hw perfevents: no interrupt-affinity property for /pmu, guessing.

Signed-off-by: Esben Haabendal <eha@deif.com>
[RV: adapt commit log to the warning emitted in current mainline]
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>