atomic-long: Fix warnings on arm64

Several inline functions in this file reference undefined functions in
U-Boot. For example:

atomic-long.h:73:9: warning: implicit declaration of function
'atomic64_sub_and_test'
atomic-long.h:80:9: warning: implicit declaration of function
'atomic64_dec_and_test'
atomic-long.h:87:9: warning: implicit declaration of function
'atomic64_inc_and_test'

Handle this the same as the 32 bit build by wrapping these functions in
a __UBOOT__ check.

Signed-off-by: Bradley Bolen <bradleybolen@gmail.com>

ubifs: Import atomic_long operations from Linux

This commit is a preperation for a subsequent UBIFS commit
which needs atomic_long operations.

Therefor "include/asm-generic/atomic-long.h" is imported
from 1860e37 Linux 3.15

Signed-off-by: Anton Habegger <anton.habegger@gmail.com>