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