common: cmd_elf.c: use uintptr_t for casts from u32 to void*

common: cmd_elf.c: use uintptr_t for casts from u32 to void*

This fixes warnings when compiling with ELDK-5.2.1 for MIPS64:

cmd_elf.c: In function 'load_elf_image_phdr':
cmd_elf.c:289:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
cmd_elf.c: In function 'load_elf_image_shdr':
cmd_elf.c:343:12: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
cmd_elf.c:346:12: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>