arm64: defconfig: enable memtest

The kernel memtest utility is incredibly useful for detecting memory
problems, but sadly isn't in defconfig.

The memtest itself is only run when the user has explicitly passed a
memtest option on the kernel command line, so simply enabling the option
should not have a negative impact.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Vladimir Murzin <vladimir.murzin@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>