arm64: defconfig: Enable CONFIG_PREEMPT by default

This way we can spot early bugs when just testing with the default
config.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/configs/defconfig

@@ -26,7 +26,7 @@ CONFIG_MODULE_UNLOAD=y
 CONFIG_ARCH_VEXPRESS=y
 CONFIG_ARCH_XGENE=y
 CONFIG_SMP=y
-CONFIG_PREEMPT_VOLUNTARY=y
+CONFIG_PREEMPT=y
 CONFIG_CMDLINE="console=ttyAMA0"
 # CONFIG_CORE_DUMP_DEFAULT_ELF_HEADERS is not set
 CONFIG_COMPAT=y