LinuxKPI: add devm_ioremap()

Given we do not seem to support ioremap() do not support the "devm"
version either and simply return NULL, which means we do not have
to keep track of the memory to be freed on device free later.

Sponsored by: The FreeBSD Foundation
MFC after: 10 days
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D40173