soc: mec172x: ecia: Adjust girq_regs to avoid flexible-array-like behaviors

Struct girq_regs has an array field of length 1 at its end. Since this
field is never used in the code base, we can replace it with a non-array
field of its original element type to remove the flexible-array-like
behaviors and avoid using a flexible array file in the middle of a
struct or defining an array of flexible array struct type.

Fixes zephyrproject-rtos#84251

Signed-off-by: Ella MA <xutong.ma@inria.fr>