metag: Time keeping

Add time keeping code for metag. Meta hardware threads have 2 timers.
The background timer (TXTIMER) is used as a free-running time base, and
the interrupt timer (TXTIMERI) is used for the timer interrupt. Both
counters traditionally count at approximately 1MHz.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>

drivers/clocksource/Kconfig

@@ -58,3 +58,8 @@ config CLKSRC_ARM_GENERIC
 	def_bool y if ARM64
 	help
 	  This option enables support for the ARM generic timer.
+
+config CLKSRC_METAG_GENERIC
+	def_bool y if METAG
+	help
+	  This option enables support for the Meta per-thread timers.