cfi_flash: Use uintptr_t for casts from u32 to void *

This fixes this build warning:

Configuring for qemu_mips64 - Board: qemu-mips64, Options: SYS_BIG_ENDIAN
   text    data     bss     dec     hex filename
 215344   13082  218720  447146   6d2aa qemu_mips64/u-boot
cfi_flash.c: In function 'flash_map':
cfi_flash.c:217:9: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Tom Rini <trini@ti.com>