<drivers/pin.h> needed in each module's file.

[majianjia]

After excluded #include <at_socket.h> as mentioned in #171

Failed to compile with codes that related to pin driver.
For example:

at_device/class/sim800c/at_device_sim800c.c

Line 880 in 4d01865

rt_pin_mode(sim800c->power_pin, PIN_MODE_OUTPUT);

will post errors and warnings that cannot find these functions and macros.

By adding #include <drivers/pin.h> solve this problem.
Should you consider adding the pin header for all classes.

But this is now a dependency of the pin driver. Should this be added to the Kconfig?

If we don't want the dependency, should it provide an interface or a function pointer for users to customize their own power on/power off method? Because sometimes, people are not using a native pin to control the power instead using an I2C IO expansion chip.