hv/mod_timer: make timer into an arch-independent module

x86/timer.[ch] was moved to the common directory largely unchanged.

x86 specific code now resides in x86/tsc_deadline_timer.c and its
interface was defined in hw/hw_timer.h. The interface defines two
functions: init_hw_timer() and set_hw_timeout() that provides HW
specific initialization and timer interrupt source.

Other than these two functions, the timer module is largely arch
agnostic.

Tracked-On: #5920
Signed-off-by: Rong Liu <rong2.liu@intel.com>
Reviewed-by: Jason Chen CJ <jason.cj.chen@intel.com>

hypervisor/arch/x86/cpu.c

@@ -32,6 +32,7 @@
 #include <ivshmem.h>
 #include <asm/rtcm.h>
 #include <reloc.h>
+#include <asm/tsc.h>
 #include <ticks.h>
 #include <delay.h>