ramips: move set-irq-affinity script to mt7621 subtarget

Move the set-irq-affinity script to mt7621 because it is the only
SMP subtarget.

Signed-off-by: DENG Qingfang <dengqf6@mail2.sysu.edu.cn>

diff --git a/target/linux/ramips/base-files/etc/init.d/set-irq-affinity b/target/linux/ramips/mt7621/base-files/etc/init.d/set-irq-affinity
similarity index 100%
rename from target/linux/ramips/base-files/etc/init.d/set-irq-affinity
rename to target/linux/ramips/mt7621/base-files/etc/init.d/set-irq-affinity