clocksource/drivers/timer-ti-dm: Simplify register reads with dmtimer_read()

clocksource/drivers/timer-ti-dm: Simplify register reads with dmtimer_read()

We can simplify register read access by checking for the register write
posted mode in the read function. This way we can combine the functions
for __omap_dm_timer_read() and omap_dm_timer_read_reg() into a single
function dmtimer_read().

We update the shared register access first, the timer revision specific
register access will be updated in a later patch.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Link: https://lore.kernel.org/r/20220815131250.34603-3-tony@atomide.com
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>