arm64: kdump: Fix elfcorehdr overlap caused by reserved memory processing reorder

Commit 8a6e02d ("of: reserved_mem: Restructure how the reserved
memory regions are processed") changed the processing order of reserved
memory regions, causing elfcorehdr to overlap with dynamically allocated
reserved memory regions during kdump kernel boot.

The issue occurs because:
1. kexec-tools allocates elfcorehdr in the last crashkernel reserved
   memory region and passes it to the second kernel
2. The problematic commit moved dynamic reserved memory allocation
   (like bman-fbpr) to occur during fdt_scan_reserved_mem(), before
   elfcorehdr reservation in fdt_reserve_elfcorehdr()
3. bman-fbpr with 16MB alignment requirement can get allocated at
   addresses that overlap with the elfcorehdr location
4. When fdt_reserve_elfcorehdr() tries to reserve elfcorehdr memory,
   overlap detection identifies the conflict and skips reservation
5. kdump kernel fails with "Unable to handle kernel paging request"
   because elfcorehdr memory is not properly reserved

The boot log:
Before 8a6e02d:
  OF: fdt: Reserving 1 KiB of memory at 0xf4fff000 for elfcorehdr
  OF: reserved mem: 0xf3000000..0xf3ffffff bman-fbpr

After 8a6e02d:
  OF: reserved mem: 0xf4000000..0xf4ffffff bman-fbpr
  OF: fdt: elfcorehdr is overlapped

Fix this by ensuring elfcorehdr reservation occurs before dynamic
reserved memory allocation.

Fixes: 8a6e02d ("of: reserved_mem: Restructure how the reserved memory regions are processed")
Signed-off-by: Jianpeng Chang <jianpeng.chang.cn@windriver.com>
Link: https://patch.msgid.link/20251205015934.700016-1-jianpeng.chang.cn@windriver.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

Author: Jianpeng Chang

drivers/of/fdt.c

@@ -503,8 +503,8 @@ void __init early_init_fdt_scan_reserved_mem(void)
 	if (!initial_boot_params)
 		return;
 
-	fdt_scan_reserved_mem();
 	fdt_reserve_elfcorehdr();
+	fdt_scan_reserved_mem();
 
 	/* Process header /memreserve/ fields */
 	for (n = 0; ; n++) {