So, the datasheet (something everyone asking a question relating to a specific component should link to) mentions this on p. 1358 under "Electrical Characteristics":
V_in_GPIO: Input voltage on GPIOs, regardless of whether the microcontroller is powered: min = -0.3V max = +5.5V
In other words, you should be fine. Notice that you'd want to have a low-pass filter on the input, anyway for obvious reasons, so something like
simulate this circuit – Schematic created using CircuitLab
Notice that there's no voltage clamping diode here anymore – simply because the voltage follower Opamp isn't able to go below 0V or above its input voltage.
Also note that the values of the low pass filter components are something you must chose to match your signal / your sampling rate – as hinted above, it's absolutely necessary to limit the bandwidth of any signal prior to digitization. Otherwise, aliasing happens. Chose one of the many available RC LPF design tools to determine sensible values. I can also recommend analog devices' Filter Design Wizard, which allows you to create active, op-amp based filters with gain in a single step. Really neat!