fsl: ls102x: remove redundant GENERIC_TIMER_CLK

Some Freescale boards used an extra version of the constant to hold the
Generic Timer frequency. This can easily be covered by the now unified
COUNTER_FREQUENCY constant, so remove this extra variable from those
boards.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: York Sun <york.sun@nxp.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>