stm32:L4:flash: support erasing pages on bank 2

Reviewed-by: Karl Palsson <karlp@tweak.net.au> (fixed code style)

lib/stm32/l4/flash.c

@@ -160,6 +160,9 @@ void flash_erase_page(uint32_t page)
 
 	/* page and bank are contiguous bits */
 	FLASH_CR &= ~((FLASH_CR_PNB_MASK << FLASH_CR_PNB_SHIFT) | FLASH_CR_BKER);
+	if (page > 255)	{
+		FLASH_CR |= FLASH_CR_BKER;
+	}
 	FLASH_CR |= page << FLASH_CR_PNB_SHIFT;
 	FLASH_CR |= FLASH_CR_PER;
 	FLASH_CR |= FLASH_CR_START;