drivers: gpio: add interrupt support for the CH32V family

The WCH GPIO peripheral integrates with the EXTI and supports firing
interrupts when a GPIO pin changes.

Add optional support for firing a callback on rising edge, falling
edge, or both edges.

Tested on the `linkw` and the `ch32v006evt` using
`samples/basic/button`.

Signed-off-by: Michael Hope <michaelh@juju.nz>