add serial_ar933x.h

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
serial_ar933x.c: start baudrate raleted work and set conig register

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
ar933x.c: continue on rate related work

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
ath79.h: add ar933x_*_rate

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>