cmd: Move meminfo command into its own file

In preparation for expanding this command, move it into a separate file.
Rename the function to remove the extra underscore. Update the number of
arguments to 1, since 3 is incorrect.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/cmd/meminfo.c b/cmd/meminfo.c
new file mode 100644
index 0000000..bb9bcec
--- /dev/null
+++ b/cmd/meminfo.c
@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright 2024 Google LLC
+ * Written by Simon Glass <sjg@chromium.org>
+ */
+
+#include <command.h>
+#include <display_options.h>
+#include <asm/global_data.h>
+
+DECLARE_GLOBAL_DATA_PTR;
+
+static int do_meminfo(struct cmd_tbl *cmdtp, int flag, int argc,
+		       char *const argv[])
+{
+	puts("DRAM:  ");
+	print_size(gd->ram_size, "\n");
+
+	return 0;
+}
+
+U_BOOT_CMD(
+	meminfo,	1,	1,	do_meminfo,
+	"display memory information",
+	""
+);