Servo control with 555 timer
A 555 can generate servo pulses, but to 'sweep' the pulse width you would need a second 555 (or a 556, or some other circuit that produces a triangle wave). Then to smooth the 50Hz pulse frequency sufficiently for accurate A/D conversion you need to integrate it with a long time constant, so the response time will be slow.

If you still have a digital output available then it might be easier (and cheaper!) to just generate the servo pulse in software. Set up a 20ms timer interrupt and create the 1~2ms pulse using a software delay loop inside the ISR. 

The servo pulse timing has to be accurate to within a few microseconds, but most servos can handle quite wide variation in repetition rate (at least 15~25ms) so the 20ms timer doesn't have have to be precise. If for some reason you can't use interrupts you might still be able produce pulses with the required repetition rate in the foreground, eg. by padding out your main loop execution time to ~20ms.