Rename pte.h to pte-v4.h and start including directly either pte-v4.h

Rename pte.h to pte-v4.h and start including directly either pte-v4.h
or pte-v6.h in files which needs it.

There are quite internal definitions in pte-v4.h and pte-v6.h headers
specific for corresponding pmap implementation. These headers should be
included only in very few files and an intention is to not hide for
which implementation such files are.

Further, sys/arm/arm/elf_trampoline.c is an example of file which
uses armv4 like pmap implementation for both armv4 and armv6 platforms.
This is another reason why pte.h which includes specific header
according to __ARM_ARCH is not created.