For kexec, we need more functionality from the IDMAP system.  We need to
be able to convert physical addresses to their identity mappped versions
as well as virtual addresses.

Convert the existing arch_virt_to_idmap() to deal with physical
addresses instead.
Acked-by: NSantosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>