drivers: litex_timer: fix usage of DT_ defines

This commit switches from using device tree automatically
generated address-based defines to the instance id-based ones.

Without this change it is not be possible to re-use the driver
on boards where the device is located at different location
than 0xe0002800.

Signed-off-by: Mateusz Holenko <mholenko@antmicro.com>