dm: spl: Support device tree when BSS is in a different section

At present in SPL we place the device tree immediately after BSS. This
avoids needing to copy it out of the way before BSS can be used. However on
some boards BSS is not placed with the image - e.g. it can be in RAM if
available.

Add an option to tell U-Boot that the device tree should be placed at the
end of the image binary (_image_binary_end) instead of at the end of BSS.

Note: A common reason to place BSS in RAM is to support the FAT filesystem.
We should update the code so that it does not use so much BSS.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

Kconfig

@@ -152,6 +152,16 @@ config SPL_STACK_R_MALLOC_SIMPLE_LEN
 	  to give board_init_r() a larger heap then the initial heap in
 	  SRAM which is limited to SYS_MALLOC_F_LEN bytes.
 
+config SPL_SEPARATE_BSS
+	depends on SPL
+	bool "BSS section is in a different memory region from text"
+	help
+	  Some platforms need a large BSS region in SPL and can provide this
+	  because RAM is already set up. In this case BSS can be moved to RAM.
+	  This option should then be enabled so that the correct device tree
+	  location is used. Normally we put the device tree at the end of BSS
+	  but with this option enabled, it goes at _image_binary_end.
+
 config TPL
 	bool
 	depends on SPL && SUPPORT_TPL