arch/arm/samv7/sam_progmem: insert DMB instruction into data write loop

This change fix the regression that was introduced with
https://github.com/apache/incubator-nuttx/pull/4904

In case if D-Cache in configured in Write-Through mode there is
Cortex-M7 erata 1313001 that describes a situation when linefill
buffer or cache contains stale data. Even if progmem write loop
does not fully matches the description there is a possibility
to program stale data if there is no DMB instruction after each
write operation to progmem latch buffer.

Signed-off-by: Petro Karashchenko <petro.karashchenko@gmail.com>

arch/arm/src/samv7/sam_progmem.c

@@ -582,6 +582,10 @@ ssize_t up_progmem_write(size_t address, const void *buffer, size_t buflen)
       for (i = 0; i < SAMV7_PAGE_WORDS; i++)
         {
           *dest++ = *src++;
+
+#ifdef CONFIG_ARMV7M_DCACHE_WRITETHROUGH
+          ARM_DMB();
+#endif
         }
 
       /* Flush the data cache to memory */