asm-generic/mmu.h: Add support for FDPIC

No-MMU architectures often have support for FDPIC binaries. FDPIC support
requires two additional fields in the mm_context_t struct. This patch adds these
fields to the generic mm_context_t definition if support for FDPIC binaries is
enabled. This allows to use the generic mmu.h for a few more architectures.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>