drivers: ht16k33: convert keyscan driver from gpio API to kscan API

Convert the keyscan portion of the Holtek HT16K33 driver to adhere to
the kscan API instead of the GPIO API.

When this driver was introduced the kscan API was not present. The
keyscan driver was therefore implemented as a GPIO interrupt driver.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>