ARM: 8951/1: Fix Kexec compilation issue.

To perform the reserve_crashkernel() operation kexec uses SECTION_SIZE to
find a memblock in a range.
SECTION_SIZE is not defined for nommu systems. Trying to compile kexec in
these conditions results in a build error:

  linux/arch/arm/kernel/setup.c: In function ‘reserve_crashkernel’:
  linux/arch/arm/kernel/setup.c:1016:25: error: ‘SECTION_SIZE’ undeclared
     (first use in this function); did you mean ‘SECTIONS_WIDTH’?
             crash_size, SECTION_SIZE);
                         ^~~~~~~~~~~~
                         SECTIONS_WIDTH
  linux/arch/arm/kernel/setup.c:1016:25: note: each undeclared identifier
     is reported only once for each function it appears in
  linux/scripts/Makefile.build:265: recipe for target 'arch/arm/kernel/setup.o'
     failed

Make KEXEC depend on MMU to fix the compilation issue.

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