soc/ti/am335x: Map useable RAM

Maps the useable RAM so that it can be used for booting a payload.

TEST: Booted a simple ELF payload (that just flashes LEDs) on the
Beaglebone Black.

Change-Id: I7f657c97e4753071c90ba8ca800a96108807e6b9
Signed-off-by: Sam Lewis <sam.vr.lewis@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44388
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/ti/am335x/soc.c

@@ -1,7 +1,15 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
 #include <device/device.h>
+#include <symbols.h>
+#include <bootmem.h>
+
+static void soc_enable(struct device *dev)
+{
+	ram_resource(dev, 0, (uintptr_t)_dram / KiB, CONFIG_DRAM_SIZE_MB * MiB / KiB);
+}
 
 struct chip_operations soc_ti_am335x_ops = {
 	CHIP_NAME("TI AM335X")
+	.enable_dev = soc_enable,
 };