Input: driver for Adafruit Seesaw Gamepad

Adds a driver for a mini gamepad that communicates over i2c, the gamepad
has bidirectional thumb stick input and six buttons.

The gamepad chip utilizes the open framework from Adafruit called 'Seesaw'
to transmit the ADC data for the joystick and digital pin state for the
buttons. I have only implemented the functionality required to receive the
thumb stick and button state.

Steps in reading the gamepad state over i2c:
1. Reset the registers
2. Set the pin mode of the pins specified by the `BUTTON_MASK` to input
`BUTTON_MASK`: A bit-map for the six digital pins internally
connected to the joystick buttons.
3. Enable internal pullup resistors for the `BUTTON_MASK`
4. Bulk set the pin state HIGH for `BUTTON_MASK`
5. Poll the device for button and joystick state done by:
`seesaw_read_data(struct i2c_client *client, struct seesaw_data *data)`

Product page:
https://www.adafruit.com/product/5743
Arduino driver:
https://github.com/adafruit/Adafruit_Seesaw

Driver tested on RPi Zero 2W

Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Anshul Dalal <anshulusr@gmail.com>
Link: https://lore.kernel.org/r/20240106015111.882325-2-anshulusr@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>