linux/list.h: drop use of prefetch()

linux/list.h: drop use of prefetch()

The use of prefetch() in these list helpers was dropped back in 2011
in linux (e66eed651fd1 ("list: remove prefetching from regular list
iterators")). No arch in U-Boot defines any actual prefetch(), and as
the referenced commit says, it's usually not a win anyway.

Whole-sale sync of list.h is not really feasible, but we can
synchronize the macros containing a prefetch() with their linux
implementations as of v6.15-rc5, also importing the various helpers
needed, e.g. list_is_head() and list_next_entry().

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>