rt2x00: add MediaTek/RaLink Rt3352 WiSoC

Support for the RT3352 WiSoC was developed for and tested with the ALL5002
devboard running OpenWrt. For now, this supports only devices with internal
TXALC. Corrections were made according to the remarks of Stanislaw Gruszka and
Gertjan van Wingerde, thank you guys for reviewing!

Signed-off-by: Daniel Golle <dgolle@allnet.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>