LinuxKPI: io.h constify arguments and add more functions

Constify "*from" arguments and add ioread32_copy() and
ioread64_copy() based on the already existing implementations.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D36657

(cherry picked from commit 046b82842c61c2d5d1648b5024958827e0a85725)