feat(epicardium): Add a simple sleep() API call

This can be used by the second core to signal that the system can go to
a light sleep to recuce power consumption.

epicardium/drivers/sleep.c

@@ -207,3 +207,8 @@ void sleep_deepsleep(void)
 	SystemCoreClockUpdate();
 	MAX77650_setEN_SBB2(0b110);
 }
+
+void epic_sleep(uint32_t ms)
+{
+	vTaskDelay(ms);
+}

epicardium/epicardium.h

@@ -33,6 +33,7 @@ typedef _Bool bool;
 #define API_SYSTEM_EXEC             0x2
 #define API_SYSTEM_RESET            0x3
 #define API_BATTERY_VOLTAGE         0x4
+#define API_SLEEP                   0x5
 
 #define API_INTERRUPT_ENABLE        0xA
 #define API_INTERRUPT_DISABLE       0xB
@@ -335,6 +336,8 @@ API(API_SYSTEM_EXEC, int __epic_exec(char *name));
  */
 API(API_SYSTEM_RESET, void epic_system_reset(void));
 
+API(API_SLEEP, void epic_sleep(uint32_t ms));
+
 /**
  * PMIC API
  * ===============