[nrf fromlist] arch: arm: cortex_m: pm_s2ram: fix system_off rv as rv of s2ram_suspend

The return value of the system_off call is overwritten by the the call to
pm_s2ram_mark_check_and_clear. As arch_pm_s2ram_suspend needs to specify
why system_off failed, we need to make sure the rv of system_off is moved
to a safe register before calling pm_s2ram_mark_check_and_clear and moved
to r0 als rv of arch_pm_s2ram_suspend when the suspend call exits.

Upstream PR #: 80642

Signed-off-by: Hessel van der Molen <[email protected]>
(cherry picked from commit a93ae9d)

Author: HesselM

arch/arm/core/cortex_m/pm_s2ram.S

@@ -85,14 +85,19 @@ SECTION_FUNC(TEXT, arch_pm_s2ram_suspend)
 	 * not successful (in r0 the return value).
 	 */
 
+	/* Move return value of system_off to callee-saved register. */
+	mov 	r4, r0
+
 	/*
 	 * Reset the marking of suspend to RAM, return is ignored.
 	 */
 	mov 	r1, lr
 	bl	pm_s2ram_mark_check_and_clear
 	mov 	lr, r1
 
-	/* Move system_off back to r0 as return value */
+	/* Move the stored return value of system_off back to r0,
+	 * setting it as return value for this function.
+	 */
 	mov	r0, r4
 
 	pop	{r4-r12, lr}