alpha: add io{read,write}{16,32}be functions

These functions are used in some PCI drivers with big-endian
MMIO space.

Admittedly it is almost certain that no one this side of the
Moon would use such a card in an Alpha but it does get us
closer to being able to build allyesconfig or allmodconfig,
and it enables the Debian default generic config to build.

Tested-by: Raúl Porcel <armin76@gentoo.org>
Signed-off-by: Michael Cree <mcree@orcon.net.nz>
Signed-off-by: Matt Turner <mattst88@gmail.com>