uboot-sunxi: add missing type __u64

uboot-sunxi: add missing type __u64

Non Linux systems e.g. macOS lack the __u64 type and produce build errors:
In file included from tools/aisimage.c:9:
In file included from include/image.h:19:
In file included from ./arch/arm/include/asm/byteorder.h:29:
In file included from include/linux/byteorder/little_endian.h:13:
include/linux/types.h:146:9: error: unknown type name '__u64'; did you mean '__s64'?
typedef __u64 __bitwise __le64;

Resolved by declaring __u64 in include/linux/types.h
Build tested on macOS and Ubuntu.

Signed-off-by: Georgi Valkov <gvalkov@abv.bg>