Flash driver for em357

Hi,

I took the stm32x NOR flash driver and adapted it for the Ember EM357
chip. This chip is very similar to em351 and stm32w so the driver
should be easily extended to support those as well if anyone can get
their hands on some of those for testing.

changelog:
Added NOR flash driver em357

Best regards,
Erik Botö

src/flash/nor/drivers.c

@@ -41,6 +41,7 @@ extern struct flash_driver avr_flash;
 extern struct flash_driver faux_flash;
 extern struct flash_driver virtual_flash;
 extern struct flash_driver stmsmi_flash;
+extern struct flash_driver em357_flash;
 
 /**
  * The list of built-in flash drivers.
@@ -67,6 +68,7 @@ static struct flash_driver *flash_drivers[] = {
 	&faux_flash,
 	&virtual_flash,
 	&stmsmi_flash,
+	&em357_flash,
 	NULL,
 };