mm: vmscan: support setting of kswapd cpu affinity

Allow the kswapd cpu affinity to be configured.
There can be power benefits on certain targets when limiting kswapd
to run only on certain cores.

CRs-fixed: 752344
Change-Id: I8a83337ff313a7e0324361140398226a09f8be0f
Signed-off-by: Liam Mark <lmark@codeaurora.org>
[imaund@codeaurora.org: Resolved trivial context conflicts.]
Signed-off-by: Ian Maund <imaund@codeaurora.org>