drivers: timer: Disable prescalar for TI DM Timer

The timer count calculations are done with the assumption that the
prescalar is disabled. When using u-boot's SPL as an early boot stage,
the omap timer driver enables the prescalar leaving this driver to
calculate incorrect counts. So explicitly disable the prescalar to match
the driver expectation.

Signed-off-by: Abe Kohandel <abe.kohandel@gmail.com>