clocksource/drivers/ralink: Add Ralink System Tick Counter driver

clocksource/drivers/ralink: Add Ralink System Tick Counter driver

System Tick Counter is present on Ralink SoCs RT3352 and MT7620. This
driver has been in 'arch/mips/ralink' directory since the beggining of
Ralink architecture support. However, it can be moved into a more proper
place in 'drivers/clocksource'. Hence add it here adding also support for
compile test targets and reducing LOC in architecture code folder.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20241028203643.191268-2-sergio.paracuellos@gmail.com
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>