clocksource/drivers/timer-ti-dm: Remove an unused field in struct dmtimer

In "struct dmtimer", the 'rate' field is unused.
Remove it.

Found with cppcheck, unusedStructMember.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/c9f7579922c587fce334a1aa9651f3189de7a00b.1714513336.git.christophe.jaillet@wanadoo.fr