ramips: create R6220 dtsi and improve WNDR3700v5 support

ramips: create R6220 dtsi and improve WNDR3700v5 support

The R6220 and WNDR3700v5 are identical apart from using NAND/NOR flash and
having a different casing. This adds a new cleaned up R6220.dtsi with the
common bits for both devices. Both devices now have feature parity.

Performed cleanup:
 * generic DTS node names
 * regulator for usb power
 * added missing pinctrl groups
 * use switch port instead of VLAN as trigger for WAN LED

Fixes for WNDR3700v5:
 * all LEDS work
 * correct ethernet MAC addresses

Signed-off-by: Jan Hoffmann <jan@3e8.eu>