mpc5200: Add a4m2k board port

This patch adds the a4m2k MPC5200B board port. Its a derivate of
the a3m071 board with only minor changes.

Additionally this patch includes some clean-up changes:
- Remove I2C support from a3m071 as its unused
- Fix/enhance default env variables
- Fix some comments
- Add newly introduced CONFIG_SPL_TARGET to automatically build
  "u-boot-img.bin"
- Fix dtb patching in READ desciption for SPL Linux booting:
  "fdt chosen" needs to get called to patch/create the chosen node.
- Add missing call to spl_board_init():
  Define CONFIG_SPL_BOARD_INIT so that spl_board_init() will get
  called in the SPL version.

Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/board/a3m071/is46r16320d.h b/board/a3m071/is46r16320d.h
new file mode 100644
index 0000000..8183d81
--- /dev/null
+++ b/board/a3m071/is46r16320d.h
@@ -0,0 +1,35 @@
+/*
+ * (C) Copyright 2004
+ * Mark Jonas, Freescale Semiconductor, mark.jonas@motorola.com.
+ *
+ * See file CREDITS for list of people who contributed to this
+ * project.
+ *
+ * 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 program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#define SDRAM_DDR		/* is DDR */
+
+#if defined(CONFIG_MPC5200)
+/* Settings for XLB = 132 MHz */
+/* see is46r16320d datasheet and MPC5200UM chap. 8.6.1. */
+
+/* SDRAM Config Standard timing */
+#define SDRAM_MODE	0x008d0000
+#define SDRAM_EMODE	0x40010000
+#define SDRAM_CONTROL	0x70430f00
+#define SDRAM_CONFIG1	0x33622930
+#define SDRAM_CONFIG2	0x46670000
+#define SDRAM_TAPDELAY	0x10000000
+
+#else
+#error CONFIG_MPC5200 not defined
+#endif