rpi: Set bootm_size to 512MB

Set this option so that all boot images stay within the bottom 512MB of
memory. This should allow us to drop the fdt_high and initrd_high
options.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Suggested-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Christopher Obbard <christopher.obbard@linaro.org>
Tested-by: Christopher Obbard <christopher.obbard@linaro.org> # CM4 1G
diff --git a/board/raspberrypi/rpi/rpi.env b/board/raspberrypi/rpi/rpi.env
index 3022828..a327fcc 100644
--- a/board/raspberrypi/rpi/rpi.env
+++ b/board/raspberrypi/rpi/rpi.env
@@ -60,7 +60,12 @@
  * Even with the smallest possible CPU-GPU memory split of the CPU getting
  * only 64M, the remaining 25M starting at 0x02700000 should allow quite
  * large initrds before they start colliding with U-Boot.
+ *
+ * Limit bootm_size to 512MB so that all boot images stay within the bottom
+ * 512MB of memory
  */
+bootm_size=0x20000000
+
 #ifdef CONFIG_ARM64
 fdt_high=ffffffffffffffff
 initrd_high=ffffffffffffffff