power-generic: Add helper function to get_value and set_value

power-generic: Add helper function to get_value and set_value

Almost functions were implemented similarly. It means that
it is possible to be replaced with the some helper function.

When using the following get_value and set_value functions,
reduce the duplicate code and simplify the operaiont of getting/setting
value from the specific file path.

[Added function description]
- int get_value_string(char *prefix, char *res_name, char *suffix, char *buf)
- int set_value_string(char *prefix, char *res_name, char *suffix, char *buf)
- int get_value_integer(char *prefix, char *res_name, char *suffix, int *value)
- int set_value_integer(char *prefix, char *res_name, char *suffix, int value)
- int get_path(char *path, char *prefix, char *res_name, char *suffix)

Change-Id: I4031d39a1440b51caad8acaa51184c6ccbb69ecb
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>