Export ab8500_sysctrl_read() and ab8500_sysctrl_write() symobols.

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