xtensa: nommu: fix load address definitions

Change KERNELOFFSET, LOAD_MEMORY_ADDRESS and VIRTUAL_MEMORY_ADDRESS to be
relative to the start of physical memory in noMMU configuration, as
there's no uniform memory remapping to KSEG starting at 0xd0000000.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>