Add list_sort() routine from Linux

Add list_sort() routine from Linux

This pulls in Linux's list_sort.c, which is a merge sort implementation
for linked lists. Apart from adding a full featured license header and
adjusting the #include file, nothing has been changed in this code.
Define a promptless Kconfig which configurations can select when they
need this code and add it to the Makefile.

This is from Linux' lib/list_sort.c, as of commit e327fd7c8667
("lib: add module support to linked list sorting tests").

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Acked-by: Julien Grall <julien.grall@arm.com>