clocksource/drivers/tango-xtal: Rename the file for consistency

For the sake of consistency, let's rename the file to a name similar
to other file names in this directory.
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/Makefile

@@ -69,7 +69,7 @@ obj-$(CONFIG_KEYSTONE_TIMER)		+= timer-keystone.o
 obj-$(CONFIG_INTEGRATOR_AP_TIMER)	+= timer-integrator-ap.o
 obj-$(CONFIG_CLKSRC_VERSATILE)		+= timer-versatile.o
 obj-$(CONFIG_CLKSRC_MIPS_GIC)		+= mips-gic-timer.o
-obj-$(CONFIG_CLKSRC_TANGO_XTAL)		+= tango_xtal.o
+obj-$(CONFIG_CLKSRC_TANGO_XTAL)		+= timer-tango-xtal.o
 obj-$(CONFIG_CLKSRC_IMX_GPT)		+= timer-imx-gpt.o
 obj-$(CONFIG_CLKSRC_IMX_TPM)		+= timer-imx-tpm.o
 obj-$(CONFIG_ASM9260_TIMER)		+= asm9260_timer.o