ARM: 8952/1: Disable kmemleak on XIP kernels

Kmemleak relies on specific symbols to register the read only data
during init (e.g. __start_ro_after_init).
Trying to build an XIP kernel on arm results in the linking error
reported below because when this option is selected read only data
after init are not allowed since .data is read only (.rodata).

  arm-linux-gnueabihf-ld: mm/kmemleak.o: in function `kmemleak_init':
  kmemleak.c:(.init.text+0x148): undefined reference to `__end_ro_after_init'
  arm-linux-gnueabihf-ld: kmemleak.c:(.init.text+0x14c):
     undefined reference to `__end_ro_after_init'
  arm-linux-gnueabihf-ld: kmemleak.c:(.init.text+0x150):
     undefined reference to `__start_ro_after_init'
  arm-linux-gnueabihf-ld: kmemleak.c:(.init.text+0x156):
     undefined reference to `__start_ro_after_init'
  arm-linux-gnueabihf-ld: kmemleak.c:(.init.text+0x162):
     undefined reference to `__start_ro_after_init'
  arm-linux-gnueabihf-ld: kmemleak.c:(.init.text+0x16a):
     undefined reference to `__start_ro_after_init'
  linux/Makefile:1078: recipe for target 'vmlinux' failed

Fix the issue enabling kmemleak only on non XIP kernels.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>