cmd: Clean up cmd_elf a little bit

This commit cleans up cmd_elf.c per U-Boot coding convention,
and removes the unnecessary DECLARE_GLOBAL_DATA_PTR and out-of-date
powerpc comments (it actually supports not only powerpc targets).

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>