drivers: gpio: renesas_ra: Do not clear pin config in int-configure

In the current implementation, when `gpio_ra_pin_interrupt_configure`
is executed, the existing settings made by `gpio_ra_pin_configure`
are erased.
A read-modify-write method will be used to preserve the settings.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>