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>

arch/arm64/configs/defconfig

@@ -180,6 +180,7 @@ CONFIG_LOCKUP_DETECTOR=y
 # CONFIG_SCHED_DEBUG is not set
 # CONFIG_DEBUG_PREEMPT is not set
 # CONFIG_FTRACE is not set
+CONFIG_MEMTEST=y
 CONFIG_SECURITY=y
 CONFIG_CRYPTO_ANSI_CPRNG=y
 CONFIG_ARM64_CRYPTO=y