highbank: switch to use the Arm SP804 DM_TIMER driver

So far the Calxeda machines were using the CONFIG_SYS_TIMER_* macros to
simply hardcode the address of the counter register of the SP804 timer.
This method is deprecated and scheduled for removal.

Use the newly introduced SP804 DM_TIMER driver to provide timer
functionality on Highbank and Midway machines. The base address and base
frequency are taken from the devicetree.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>