timer: Add a test for timer_timebase_fallback

To test this function, sandbox CPU must set cpu_platdata.timebase_freq on
bind. It also needs to expose a method to set the current cpu. I also make
some most members of cpu_sandbox_ops static.

On the timer side, the device tree property
sandbox,timebase-frequency-fallback controls whether sandbox_timer_probe
falls back to time_timebase_fallback or to SANDBOX_TIMER_RATE.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/include/asm/cpu.h b/arch/sandbox/include/asm/cpu.h
new file mode 100644
index 0000000..c97ac7b
--- /dev/null
+++ b/arch/sandbox/include/asm/cpu.h
@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (C) 2020 Sean Anderson <seanga2@gmail.com>
+ */
+
+#ifndef __SANDBOX_CPU_H
+#define __SANDBOX_CPU_H
+
+void cpu_sandbox_set_current(const char *name);
+
+#endif /* __SANDBOX_CPU_H */