stm32/sdram: Add function to enable SDRAM power-down mode.

Add power-down mode for maximum power saving while in standby mode, to be
used by a board if needed.  Also fix a typo in a comment.

ports/stm32/sdram.c

@@ -259,7 +259,7 @@ static void sdram_init_seq(SDRAM_HandleTypeDef
 
 void sdram_enter_low_power(void) {
     // Enter self-refresh mode.
-    // In self-refresh mode the SDRAM retains data with external clocking.
+    // In self-refresh mode the SDRAM retains data without 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
@@ -276,6 +276,14 @@ void sdram_leave_low_power(void) {
         (0 << 9U));                                                  // Mode Register Definition
 }
 
+void sdram_enter_power_down(void) {
+    // Enter power-down mode.
+    FMC_SDRAM_DEVICE->SDCMR |= (FMC_SDRAM_CMD_POWERDOWN_MODE |       // Command Mode
+        FMC_SDRAM_CMD_TARGET_BANK |                                  // Command Target
+        (0 << 5U) |                                                  // Auto Refresh Number -1
+        (0 << 9U));                                                  // Mode Register Definition
+}
+
 #if __GNUC__ >= 11
 // Prevent array bounds warnings when accessing SDRAM_START_ADDRESS as a memory pointer.
 #pragma GCC diagnostic push

ports/stm32/sdram.h

@@ -13,5 +13,6 @@ void *sdram_start(void);
 void *sdram_end(void);
 void sdram_enter_low_power(void);
 void sdram_leave_low_power(void);
+void sdram_enter_power_down(void);
 bool sdram_test(bool exhaustive);
 #endif // __SDRAM_H__