[ARM] unconditionally define __virt_to_phys and __phys_to_virt

There is no machine class overriding this.  If non linear translations
are implemented again for some machines then this could be restored at
that time.

Signed-off-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>