ARM: ep93xx: use ARM_PATCH_PHYS_VIRT correctly

Just like ARCH_MULTIPLATFORM, we want to use ARM_PATCH_PHYS_VIRT
when possible, but that fails for NOMMU or XIP_KERNEL configurations.
Using 'imply' instead of 'select' gets this right and only uses
the symbol when we don't have to hardcode the address anyway.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>