drivers: interrupt_controller: add a WCH EXTI external interrupt driver

The WCH External Trigger and Interrupt controller (EXTI) supports
between 8 and 22 lines where each line can trigger an interrupt on
rising edge, falling edge, or both edges. Lines are assigned to a
group, and each group has a separate interrupt. On the CH32V003/6,
there is one group of 8 lines, while on the CH32V208 there are
multiple groups with between one and six lines per group.

In the same way as the STM32 and GD32, define an EXTI driver that
configures the peripheral and an internal interface that can configure
individual lines.

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