Export ab8500_sysctrl_read() and ab8500_sysctrl_write() symobols.

They will be used by LTP test cases.
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Reviewed-by: Bengt JONSSON <bengt.g.jonsson@stericsson.com>