drivers: clock_control: litex: remove redundant entry

remove litex,sys-clock-frequency from litex,clk,
because we already define that in the clock-frequency of cpu0.
This can be accessed via
CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC.

Signed-off-by: Fin Maaß <f.maass@vogl-electronic.com>