linuxkpi: Define ioremap_cache()

It uses the VM_MEMATTR_WRITE_BACK flag on FreeBSD.

It replaces ioremap_wb() which doesn't exist in Linux. Perhaps it
existed in the past and was removed.

Reviewed by: emaste, manu
Approved by: emaste, manu
Differential Revision: https://reviews.freebsd.org/D37916

(cherry picked from commit ed53e350d703b9e3ab78d137fd59a5a9dfef8b3f)