linux/types.h: add typedef of uintptr_t
Add this typedef in the same place as in Linux. This is necessary
to refactor libfdt inclusion.
U-Boot also defines it in include/compiler.h. Of course it should
not do that, but I do not want to open a can of worms.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>