irqchip/stm32-exti: Simplify irq description table

Having removed the event trigger type from struct stm32_desc_irq
makes worthless keep using a struct.

Replace the struct by a single dimension array and use 8 bit type
to reduce the overal memory footprint.
On armv7a this patch reduces by 7% the size of the driver, from
   text    data     bss     dec     hex filename
   6977     424       4    7405    1ced irq-stm32-exti.o
to
   6449     424       4    6877    1add irq-stm32-exti.o

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220606162757.415354-7-antonio.borneo@foss.st.com