rt2x00: Use ioremap for SoC devices instead of KSEG1ADDR.

Make the code a bit more portable to architectures that do not support
KSEG1ADDR.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Tested-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>