irqchip/stm32-exti: Convert driver to standard PM

All driver's dependencies for suspend/resume have been fixed long
ago. There are no more reasons to use syscore PM for the part of this
driver related to Cortex-A MPU.

Switch to standard PM using NOIRQ_SYSTEM_SLEEP_PM_OPS, so all the registers
of the interrupt controller get resumed before any irq gets enabled.

A side effect of this change is to drop the only global variable
'stm32_host_data', used to keep the driver's data for syscore_ops.  This
makes the driver ready to support multiple EXTI instances.

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240415134926.1254428-5-antonio.borneo@foss.st.com