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

elfcorehdr_addr is fixed address passed to Second kernel which may be conflicted
with potential reserved memory in Second 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: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220128042321.15228-1-nikhil.gupta@nxp.com