rt2x00:Add RT5372 chipset support

Signed-off-by: John Li <chen-yang.li@mediatek.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>