xtensa: clean up include/asm/vectors.h

- drop unused PHYSICAL_MEMORY_ADDRESS and XC_PADDR
- fix NMI_VECTOR_VADDR and INTLEVEL7_VECTOR_VADDR definitions: there
  should be no XCHAL_ prefix in them;
- fix the following warning seen with gcc-4.8.1:

  arch/xtensa/include/asm/vectors.h:71:5: warning: "XCHAL_HAVE_VECBASE" is not defined [-Wundef]

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>