Xtensa improvements for 4.8:
- add new kernel memory layouts for MMUv3 cores: with 256MB and 512MB
KSEG size, starting at physical address other than 0;
- make kernel load address configurable;
- clean up kernel memory layout macros;
- drop sysmem early allocator and switch to memblock;
- enable kmemleak and memory reservation from the device tree;
- wire up new syscalls: userfaultfd, membarrier, mlock2, copy_file_range,
preadv2 and pwritev2.