irqchip/stm32: Use a platform driver for stm32mp1-exti device

This irqchip driver uses the hwspinlock framework (coprocessor HW regs
access concurrency) for the stm32mp1-exti device.
Hence, this driver needs to handle the hwspinlock driver dependency
using the deferred probe mechanism which requires to move this driver
into a platform one with a probe() ops.
This applies only for the device which is "st,stm32mp1-exti" compatible,
the management of the other devices (st,stm32h7-exti / st,stm32-exti) is
kept unchanged (use IRQCHIP_DECLARE)
Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>