xtensa: get rid of __ARCH_USE_5LEVEL_HACK

xtensa has 2-level page tables and already uses pgtable-nopmd for page
table folding.

Add walks of p4d level where appropriate and drop usage of
__ARCH_USE_5LEVEL_HACK.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Message-Id: <1572964400-16542-3-git-send-email-rppt@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com> [fix up
               arch/xtensa/include/asm/fixmap.h and
               arch/xtensa/mm/tlb.c]
