clocksource: timer-keystone: introduce clocksource driver for Keystone

Add broadcast clock-event device for the Keystone arch.

The timer can be configured as a general-purpose 64-bit timer,
dual general-purpose 32-bit timers. When configured as dual 32-bit
timers, each half can operate in conjunction (chain mode) or
independently (unchained mode) of each other.

Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Santosh shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>