[ioapi] Move PAGE_SHIFT to bits/io.h

The PAGE_SHIFT definition is an architectural property, rather than an
aspect of a particular I/O API implementation (of which, in theory,
there may be more than one per architecture).

Reflect this by moving the definition to the top-level bits/io.h for
each architecture.

Signed-off-by: Michael Brown <mcb30@ipxe.org>