arm_fpga: Read generic timer counter frequency from DT

arm_fpga: Read generic timer counter frequency from DT

The ARM Generic Timer DT binding describes an (optional) property to
declare the counter frequency. Its usage is normally discouraged, as the
value should be read from the CNTFRQ_EL0 system register.

However in our case we can use it to program this register in the first
place, which avoids us to hard code a counter frequency into the code.
We keep some default value in, if the DT lacks that property for
whatever reason.

Change-Id: I5b71176db413f904f21eb16f3302fbb799cb0305
Signed-off-by: Andre Przywara <andre.przywara@arm.com>