drivers: misc: Update renesas ra external interrupt to support RA8P1

This is temporary update because in HWM only mention IRQCR but
FSP is defining for IRQCRa and IRQCRb with same values.
Haven't understood why need both  IRQCRa and IRQCRb.
Current update is using IRACRa.

Signed-off-by: Thao Luong <thao.luong.uw@renesas.com>