char/agp: introduce asm-generic/agp.h

There are several architectures that duplicate definitions of
map_page_into_agp(), unmap_page_from_agp() and flush_agp_cache().

Define those in asm-generic/agp.h and use it instead of duplicated
per-architecture headers.

Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>