Revert "[ARM] unconditionally define __virt_to_phys and __phys_to_virt"

This reverts commit 75f4aa15.

We have a couple of platforms which require non-linear P:V mappings,
so we need these to be overridable.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>