ARM: tegra: fix COUNTER_FREQUENCY for T210

While T210 boards all have 38.4MHz crystals, per the TRM, the only
supported configuration is to divide the crystal frequency by 2 to
generate clk_m, which is what feeds the ARM generic timers amongst other
things. Fix the value of COUNTER_FREQUENCY to reflect this divide-by-2.

When I queried the 19.2 value in Tom's original T210 patches, I wasn't
aware of this extra divide-by-2, and didn't notice any effect from the
incorrect value, since its only used if U-Boot is booted in EL3, whereas
I'm booting it in EL2.

Reported-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>