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 #84251

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

Author: Snape3058

soc/microchip/mec/mec172x/reg/mec172x_ecia.h

@@ -1132,7 +1132,7 @@ struct girq_regs {
 	volatile uint32_t EN_SET;
 	volatile uint32_t RESULT;
 	volatile uint32_t EN_CLR;
-	uint32_t RSVD1[1];
+	volatile uint32_t RSVD1;
 };
 
 /** @brief ECIA registers with each GIRQ elucidated */