LF-5222 of/fdt: move elfcorehdr reservation early for crash dump kernel

commit fbb318dfdfcf9589f090d025174dbc55ecd16413 from
https://source.codeaurora.org/external/imx/linux-imx

elfcorehdr_addr is fixed address passed to 2nd kernel which may be conflicted
with potential reserved memory in 2nd kernel,so fdt_reserve_elfcorehdr() ahead
of fdt_init_reserved_mem() can relieve this situation.

Signed-off-by: Nikhil Gupta <nikhil.gupta@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>