frv: move HAVE_ARCH_UNMAPPED_AREA to pgtable.h

it has no business in uaccess.h, everything else has it in pgtable.h and
the only user (mm/mmap.c) unconditionally pulls asm/pgtable.h via linux/mm.h.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>