Make basic Broadcom I/O space reading functions work

It's now possible to use Broadcom functions to read the I/O registers of
SDIO card. The functions were copied from the BSD-licensed Broadcom Linux driver
as-is. To make it possible, a small Linux compatibility layer was introduced.

Currently the card responds with the correct version number ("magic")
when reading the corresponding address.

Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D12111