stm32/sdram: Add SDRAM enter/leave self-refresh mode functions.

These functions enable SDRAM data retention in stop mode.  Example usage,
in mpconfigboard.h:

    #define MICROPY_BOARD_ENTER_STOP sdram_enter_low_power();
    #define MICROPY_BOARD_LEAVE_STOP sdram_leave_low_power();

ports/stm32/sdram.c

@@ -49,8 +49,7 @@
 
 #ifdef FMC_SDRAM_BANK
 
-static void sdram_init_seq(SDRAM_HandleTypeDef
-    *hsdram, FMC_SDRAM_CommandTypeDef *command);
+static void sdram_init_seq(SDRAM_HandleTypeDef *hsdram, FMC_SDRAM_CommandTypeDef *command);
 extern void __fatal_error(const char *msg);
 
 bool sdram_init(void) {
@@ -254,6 +253,25 @@ static void sdram_init_seq(SDRAM_HandleTypeDef
     #endif
 }
 
+void sdram_enter_low_power(void) {
+    // Enter self-refresh mode.
+    // In self-refresh mode the SDRAM retains data with external clocking.
+    FMC_SDRAM_DEVICE->SDCMR |= (FMC_SDRAM_CMD_SELFREFRESH_MODE |     // Command Mode
+        FMC_SDRAM_CMD_TARGET_BANK |                                  // Command Target
+        (0 << 5U) |                                                  // Auto Refresh Number -1
+        (0 << 9U));                                                  // Mode Register Definition
+}
+
+void sdram_leave_low_power(void) {
+    // Exit self-refresh mode.
+    // Self-refresh mode is exited when the device is accessed or the mode bits are
+    // set to Normal mode, so technically it's not necessary to call this functions.
+    FMC_SDRAM_DEVICE->SDCMR |= (FMC_SDRAM_CMD_NORMAL_MODE |          // Command Mode
+        FMC_SDRAM_CMD_TARGET_BANK |                                  // Command Target
+        (0 << 5U) |                                                  // Auto Refresh Number - 1
+        (0 << 9U));                                                  // Mode Register Definition
+}
+
 bool sdram_test(bool fast) {
     uint8_t const pattern = 0xaa;
     uint8_t const antipattern = 0x55;

ports/stm32/sdram.h

@@ -11,5 +11,7 @@
 bool sdram_init(void);
 void *sdram_start(void);
 void *sdram_end(void);
+void sdram_enter_low_power(void);
+void sdram_leave_low_power(void);
 bool sdram_test(bool fast);
 #endif // __SDRAM_H__