ARM: socfpga: Permit overriding the default timer frequency

The default timer rate may be different than 25 MHz, permit overriding
the default rate in board configuration file. Ultimatelly, this should
be properly handled by a clock driver, however that is not available
on Gen5 yet.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Ley Foon Tan <ley.foon.tan@intel.com>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>