Current nommu_virt_defconfig can't compile:

In file included from
arch/riscv/kernel/crash_core.c:3:
arch/riscv/kernel/crash_core.c:
In function 'arch_crash_save_vmcoreinfo':
arch/riscv/kernel/crash_core.c:8:27:
error: 'VA_BITS' undeclared (first use in this function)
    8 |         VMCOREINFO_NUMBER(VA_BITS);
      |                           ^~~~~~~

Add MMU dependency for KEXEC_FILE.

Fixes: 6261586e ("RISC-V: Add kexec_file support")
Reported-by: Conor Dooley <conor.dooley@microchip.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Tested-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20221207091112.2258674-1-guoren@kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>