Merged in david_s5/nuttx/master_h7 (pull request #1025)

stm32h7: bbsram issues causing no writes and  hardfaults

* stm32_pwr:Ensure data is flushed on backup domain access changes

* stm32h7:bbsram convince compiler to perform 32 bit write

Approved-by: Gregory Nutt <gnutt@nuttx.org>

arch/arm/src/stm32h7/stm32_bbsram.c

@@ -334,7 +334,7 @@ static void stm32_bbsram_ecc_workaround(FAR struct bbsramfh_s *pf)
 {
   /* Force the last writes to be a a full 32 bit word */
 
-  pf->unit32 = (volatile uint32_t) pf->unit32;
+  *((volatile uint32_t *) &pf->unit32) = (volatile uint32_t) pf->unit32;
 }
 #else
 #define stm32_bbsram_ecc_workaround()

arch/arm/src/stm32h7/stm32_pwr.c

@@ -47,6 +47,7 @@
 #include <stdbool.h>
 #include <errno.h>
 
+#include "barriers.h"
 #include "up_arch.h"
 #include "stm32_pwr.h"
 
@@ -151,6 +152,7 @@ void stm32_pwr_enablebkp(bool writable)
   bool wait = false;
 
   flags = enter_critical_section();
+  ARM_DSB();
 
   /* Get the current state of the STM32 PWR control register */
 
@@ -186,13 +188,14 @@ void stm32_pwr_enablebkp(bool writable)
       wait = true;
     }
 
+  ARM_DSB();
   leave_critical_section(flags);
 
   if (wait)
     {
       /* Enable does not happen right away */
 
-      up_udelay(4000);
+      up_udelay(4);
     }
 }