ioremap() limitations

[penberg]

As detailed in f910abf, the ioremap() function has serious limitations:

• ioremap() doesn't doseparate virtual memory allocation. Instead, it uses the user-provided I/O memory address as the virtual address, which can cause problems if I/O memory address space overlaps with physical address space, for example.
• ioremap() does not set any appropriate cache bits for the page table entry, which can cause problems when accessing the I/O memory region (e.g. stale reads or writes not hitting I/O device because of write-back).

Both issues need to be addressed for ioremap() to be robust and work across different devices and platforms.