freertos: io: fix IAR compile error due to 'void *' variable

Fix EWARM compilation error due to void pointer usage.
This fix has already been implemented for the generic system, but
was missing for the FreeRTOS.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>

lib/system/freertos/io.c

@@ -15,10 +15,10 @@ void metal_sys_io_mem_map(struct metal_io_region *io)
 {
 	unsigned long p;
 	size_t psize;
-	void *va;
+	size_t *va;
 
 	va = io->virt;
-	psize = io->size;
+	psize = (size_t)io->size;
 	if (psize) {
 		if (psize >> io->page_shift)
 			psize = (size_t)1 << io->page_shift;