schedule: rename ll_schedule.c to ll_schedule_xtos.c

The scheduler implementation in ll_schedule.c implements a low-latency
(LL) scheduler using old XTOS interfaces. The code has been modified so
it can be used with Zephyr using the SOF rtos/ interface.

The code is only used by one build target at this point and it should
not be used for new targets. To make it clear this is not the primary
implementation, rename the file with a "xtos" postfix.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>

Author: kv2019i

src/schedule/CMakeLists.txt

@@ -29,7 +29,7 @@ if (CONFIG_SOC_ACP_6_0)
 		)
 	else()
 		zephyr_library_sources(
-			ll_schedule.c
+			ll_schedule_xtos.c
 		)
 	endif()
 else()