ath6kl: Support different uart_tx pin and refclk configuration

AR6003 family use uart_tx=8 and refclk=26Mhz by default, and AR6004 family
uses different uart_tx pin and could also support various xtal source,
moves these per hw configuration.

Signed-off-by: Ryan Hsu <ryanhsu@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>