ath79: reorganize DTSI for ar7240 TP-Link devices

The current set of TP-Link devices with ar7240 SoC all share
the same DTSI file. As the latter is very similar to the
definition required for the to-be-supported TP-Link TL-WA devices
with ar7240, this patch splits the definitions into a shared part
for all TP-Link devices (ar7240_tplink.dtsi) and a file containing
the specific setup for the present TL-WR devices
(ar7240_tplink_tl-wr.dtsi), equivalent to the former
ar7240_tplink_tl-wr74xn-v1.dtsi.

While at it, remove unused firmware partition label and rename
pinmux_switch_led_pins.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>