of: fdt: Rename reserve_elfcorehdr() to fdt_reserve_elfcorehdr()

of: fdt: Rename reserve_elfcorehdr() to fdt_reserve_elfcorehdr()

On ia64/allmodconfig:

    drivers/of/fdt.c:609:20: error: conflicting types for 'reserve_elfcorehdr'; have 'void(void)'
      609 | static void __init reserve_elfcorehdr(void)
	  |                    ^~~~~~~~~~~~~~~~~~
    arch/ia64/include/asm/meminit.h:43:12: note: previous declaration of 'reserve_elfcorehdr' with type 'int(u64 *, u64 *)' {aka 'int(long long unsigned int *, long long unsigned int *)'}
       43 | extern int reserve_elfcorehdr(u64 *start, u64 *end);
	  |            ^~~~~~~~~~~~~~~~~~

Fix this by prefixing the FDT function name with "fdt_".

Fixes: f7e7ce93aac13118 ("of: fdt: Add generic support for handling elf core headers property")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/f6eabbbce0fba6da3da0264c1e1cf23c01173999.1629884393.git.geert+renesas@glider.be
Signed-off-by: Rob Herring <robh@kernel.org>