c6x: Use generic asm/mmu.h

c6x's asm/mmu.h is basically identical to asm-generic/mmu.h, so use it instead
of the custom version.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Mark Salter <msalter@redhat.com>
Tested-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>