New WIP port: Echo R1

The Echo R1 is a new open-hardware music player design, based
on the STM32H743 microcontroller. Schematics and hardware
documentation for it can be found here:

- https://github.com/amachronic/echoplayer

This is an incomplete port. The bootloader can be loaded using
OpenOCD and it can draw to the LCD using SPI. SDRAM is working
but hasn't been extensively tested.

Change-Id: Ifd2bee15c49868fbc989683d3ca14dce48bf3e18

firmware/target/arm/stm32/system-target.h

@@ -0,0 +1,34 @@
+/***************************************************************************
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
+ * $Id$
+ *
+ * Copyright (C) 2025 Aidan MacDonald
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ****************************************************************************/
+#ifndef __STM32_SYSTEM_TARGET_H__
+#define __STM32_SYSTEM_TARGET_H__
+
+#include "system-arm.h"
+#include "cpucache-armv7m.h"
+
+/* Implemented by the target -- can be a no-op if not needed */
+void gpio_init(void) INIT_ATTR;
+void fmc_init(void) INIT_ATTR;
+
+void udelay(uint32_t us);
+void mdelay(uint32_t ms);
+
+#endif /* __STM32_SYSTEM_TARGET_H__ */