FRV: Use <asm-generic/pgtable.h> in NOMMU mode

asm-frv/pgtable.h could just #include <asm-generic/pgtable.h> in NOMMU mode
rather than #defining macros for lazy MMU and CPU stuff.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>