arm64/defconfig: Enable CONFIG_KEXEC_FILE

kexec_file_load() syscall interface is now supported for
arm64 architecture as well via commits:
3751e728 ("arm64: kexec_file: add crash dump support") and
3ddd9992

 ("arm64: enable KEXEC_FILE config")].

This patch enables config KEXEC_FILE by default in the
arm64 defconfig, so that user-space tools like kexec-tools
can use the same as the default interface for kexec/kdump
on arm64.

Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: James Morse <james.morse@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: kexec@lists.infradead.org

Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/1586212300-30797-1-git-send-email-bhsharma@redhat.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>