leds: leds-ns2: depends on ARCH_KIRKWOOD

With the DT conversion, the board Kconfig symbols MACH_ are going to be
removed. In order to prepare this removal, this patch replaces alls the
machines dependencies for leds-ns2 by ARCH_KIRKWOOD.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Bryan Wu <cooloney@gmail.com>