irqchip/stm32-exti: Tag emr register as undefined for stm32mp15

The reference manual RM0436 of stm32mp15 till version v4.0 was
erroneously reporting the Event Mask Registers (EMR) for the
Cortex-A CPUs.
These registers have been removed from v5.0 of the manual and the
corresponding offsets have been marked as 'Reserved'.

Prevent accessing these reserved addresses by tagging the EMR
offsets as UNDEF_REG and modifying the code to handle this case.

Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220606162757.415354-5-antonio.borneo@foss.st.com