fix(epicardium): Use SEV-on-pend instead of enabling IRQ

Hopefully fixes #5.

Signed-off-by: Rahix <rahix@rahix.de>

epicardium/main.c

@@ -57,6 +57,9 @@ int main(void)
 	card10_init();
 	card10_diag();
 
+	/* TODO: Move this to its own function */
+	SCB->SCR |= SCB_SCR_SEVONPEND_Msk;
+
 	pmic_set_button_callback(pmic_button);
 
 	cdcacm_init();

epicardium/support.c

@@ -24,14 +24,10 @@ void pre_idle_sleep(TickType_t xExpectedIdleTime)
 		 * TODO: Ensure this is actually correct and does not have any
 		 * race conditions.
 		 */
-		__asm volatile( "cpsie i" ::: "memory" );
 		__asm volatile( "dsb" ::: "memory" );
-		__asm volatile( "isb" );
+		__asm volatile( "sev" );
+		__asm volatile( "wfe" );
 		__asm volatile( "wfe" );
-		__asm volatile( "dsb" ::: "memory" );
-		__asm volatile( "isb" );
-		__asm volatile( "cpsid i" ::: "memory" );
-		__asm volatile( "dsb" );
 		__asm volatile( "isb" );
 	}
 }