clocksource: rename driver for ARMv8 Timer to ARM architected timer ...

The driver matches against both "arm,armv7-timer" and "arm,armv8-timer"
compatibles, thus rename it to better reflect that it's not only ARMv8
specific.

Only functional change intended is name changes. The symbol is default
y, so the rename shouldn't cause it to get lost with olddefconfig.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>