stm32: use DSB to ensure GPU clock gets enabled

src/lv_gpu/lv_gpu_stm32_dma2d.c

@@ -71,6 +71,9 @@ void lv_gpu_stm32_dma2d_init(void)
 # warning "LVGL can't enable the clock of DMA2D"
 #endif
 
+    /* Wait for hardware access to complete */
+    __asm volatile("DSB\n");
+
     /* Delay after setting peripheral clock */
     volatile uint32_t temp = RCC->AHB1ENR;