mfd: ab8500-sysctrl: Provide configuration for SysClkReqRfClkBuf registers

Add the possibility to pass configuration settings for
SysCl1kReqRfClkBuf to SysClk8ReqRfClkBuf via platform data.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Kennet Wallden <kennet.wallden@stericsson.com>
Reviewed-by: Karl-Johan PERNTZ <karl-johan.perntz@stericsson.com>
Reviewed-by: Bengt JONSSON <bengt.g.jonsson@stericsson.com>