alpha: stop using asm-generic/iomap.h

alpha: stop using asm-generic/iomap.h

Alpha has custom definitions for ioread64()/iowrite64(), which now
don't exist in the lib/iomap.c variant. This is an endless source
of build failures, since alpha tries to share a couple of function
declarations.

Change alpha to have its own prototypes that match the definitions
in arch/alpha/kerne/io.c instead.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>